sqlglot.transforms API documentation

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# sqlglot.transforms

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      1from __future__ import annotations
      2
      3import typing as t
      4
      5from sqlglot import expressions as exp
      6from sqlglot.helper import find_new_name, name_sequence
      7
      8if t.TYPE_CHECKING:
      9    from sqlglot.generator import Generator
     10
     11
     12def unalias_group(expression: exp.Expression) -> exp.Expression:
     13    """
     14    Replace references to select aliases in GROUP BY clauses.
     15
     16    Example:
     17        >>> import sqlglot
     18        >>> sqlglot.parse_one("SELECT a AS b FROM x GROUP BY b").transform(unalias_group).sql()
     19        'SELECT a AS b FROM x GROUP BY 1'
     20
     21    Args:
     22        expression: the expression that will be transformed.
     23
     24    Returns:
     25        The transformed expression.
     26    """
     27    if isinstance(expression, exp.Group) and isinstance(expression.parent, exp.Select):
     28        aliased_selects = {
     29            e.alias: i
     30            for i, e in enumerate(expression.parent.expressions, start=1)
     31            if isinstance(e, exp.Alias)
     32        }
     33
     34        for group_by in expression.expressions:
     35            if (
     36                isinstance(group_by, exp.Column)
     37                and not group_by.table
     38                and group_by.name in aliased_selects
     39            ):
     40                group_by.replace(exp.Literal.number(aliased_selects.get(group_by.name)))
     41
     42    return expression
     43
     44
     45def eliminate_distinct_on(expression: exp.Expression) -> exp.Expression:
     46    """
     47    Convert SELECT DISTINCT ON statements to a subquery with a window function.
     48
     49    This is useful for dialects that don't support SELECT DISTINCT ON but support window functions.
     50
     51    Args:
     52        expression: the expression that will be transformed.
     53
     54    Returns:
     55        The transformed expression.
     56    """
     57    if (
     58        isinstance(expression, exp.Select)
     59        and expression.args.get("distinct")
     60        and expression.args["distinct"].args.get("on")
     61        and isinstance(expression.args["distinct"].args["on"], exp.Tuple)
     62    ):
     63        distinct_cols = expression.args["distinct"].pop().args["on"].expressions
     64        outer_selects = expression.selects
     65        row_number = find_new_name(expression.named_selects, "_row_number")
     66        window = exp.Window(this=exp.RowNumber(), partition_by=distinct_cols)
     67        order = expression.args.get("order")
     68
     69        if order:
     70            window.set("order", order.pop())
     71        else:
     72            window.set("order", exp.Order(expressions=[c.copy() for c in distinct_cols]))
     73
     74        window = exp.alias_(window, row_number)
     75        expression.select(window, copy=False)
     76
     77        return (
     78            exp.select(*outer_selects, copy=False)
     79            .from_(expression.subquery("_t", copy=False), copy=False)
     80            .where(exp.column(row_number).eq(1), copy=False)
     81        )
     82
     83    return expression
     84
     85
     86def eliminate_qualify(expression: exp.Expression) -> exp.Expression:
     87    """
     88    Convert SELECT statements that contain the QUALIFY clause into subqueries, filtered equivalently.
     89
     90    The idea behind this transformation can be seen in Snowflake's documentation for QUALIFY:
     91    https://docs.snowflake.com/en/sql-reference/constructs/qualify
     92
     93    Some dialects don't support window functions in the WHERE clause, so we need to include them as
     94    projections in the subquery, in order to refer to them in the outer filter using aliases. Also,
     95    if a column is referenced in the QUALIFY clause but is not selected, we need to include it too,
     96    otherwise we won't be able to refer to it in the outer query's WHERE clause.
     97    """
     98    if isinstance(expression, exp.Select) and expression.args.get("qualify"):
     99        taken = set(expression.named_selects)
    100        for select in expression.selects:
    101            if not select.alias_or_name:
    102                alias = find_new_name(taken, "_c")
    103                select.replace(exp.alias_(select, alias))
    104                taken.add(alias)
    105
    106        outer_selects = exp.select(*[select.alias_or_name for select in expression.selects])
    107        qualify_filters = expression.args["qualify"].pop().this
    108
    109        select_candidates = exp.Window if expression.is_star else (exp.Window, exp.Column)
    110        for expr in qualify_filters.find_all(select_candidates):
    111            if isinstance(expr, exp.Window):
    112                alias = find_new_name(expression.named_selects, "_w")
    113                expression.select(exp.alias_(expr, alias), copy=False)
    114                column = exp.column(alias)
    115
    116                if isinstance(expr.parent, exp.Qualify):
    117                    qualify_filters = column
    118                else:
    119                    expr.replace(column)
    120            elif expr.name not in expression.named_selects:
    121                expression.select(expr.copy(), copy=False)
    122
    123        return outer_selects.from_(expression.subquery(alias="_t", copy=False), copy=False).where(
    124            qualify_filters, copy=False
    125        )
    126
    127    return expression
    128
    129
    130def remove_precision_parameterized_types(expression: exp.Expression) -> exp.Expression:
    131    """
    132    Some dialects only allow the precision for parameterized types to be defined in the DDL and not in
    133    other expressions. This transforms removes the precision from parameterized types in expressions.
    134    """
    135    for node in expression.find_all(exp.DataType):
    136        node.set(
    137            "expressions", [e for e in node.expressions if not isinstance(e, exp.DataTypeParam)]
    138        )
    139
    140    return expression
    141
    142
    143def unnest_to_explode(expression: exp.Expression) -> exp.Expression:
    144    """Convert cross join unnest into lateral view explode."""
    145    if isinstance(expression, exp.Select):
    146        for join in expression.args.get("joins") or []:
    147            unnest = join.this
    148
    149            if isinstance(unnest, exp.Unnest):
    150                alias = unnest.args.get("alias")
    151                udtf = exp.Posexplode if unnest.args.get("offset") else exp.Explode
    152
    153                expression.args["joins"].remove(join)
    154
    155                for e, column in zip(unnest.expressions, alias.columns if alias else []):
    156                    expression.append(
    157                        "laterals",
    158                        exp.Lateral(
    159                            this=udtf(this=e),
    160                            view=True,
    161                            alias=exp.TableAlias(this=alias.this, columns=[column]),  # type: ignore
    162                        ),
    163                    )
    164
    165    return expression
    166
    167
    168def explode_to_unnest(index_offset: int = 0) -> t.Callable[[exp.Expression], exp.Expression]:
    169    """Convert explode/posexplode into unnest."""
    170
    171    def _explode_to_unnest(expression: exp.Expression) -> exp.Expression:
    172        if isinstance(expression, exp.Select):
    173            from sqlglot.optimizer.scope import Scope
    174
    175            taken_select_names = set(expression.named_selects)
    176            taken_source_names = {name for name, _ in Scope(expression).references}
    177
    178            def new_name(names: t.Set[str], name: str) -> str:
    179                name = find_new_name(names, name)
    180                names.add(name)
    181                return name
    182
    183            arrays: t.List[exp.Condition] = []
    184            series_alias = new_name(taken_select_names, "pos")
    185            series = exp.alias_(
    186                exp.Unnest(
    187                    expressions=[exp.GenerateSeries(start=exp.Literal.number(index_offset))]
    188                ),
    189                new_name(taken_source_names, "_u"),
    190                table=[series_alias],
    191            )
    192
    193            # we use list here because expression.selects is mutated inside the loop
    194            for select in list(expression.selects):
    195                explode = select.find(exp.Explode)
    196
    197                if explode:
    198                    pos_alias = ""
    199                    explode_alias = ""
    200
    201                    if isinstance(select, exp.Alias):
    202                        explode_alias = select.args["alias"]
    203                        alias = select
    204                    elif isinstance(select, exp.Aliases):
    205                        pos_alias = select.aliases[0]
    206                        explode_alias = select.aliases[1]
    207                        alias = select.replace(exp.alias_(select.this, "", copy=False))
    208                    else:
    209                        alias = select.replace(exp.alias_(select, ""))
    210                        explode = alias.find(exp.Explode)
    211                        assert explode
    212
    213                    is_posexplode = isinstance(explode, exp.Posexplode)
    214                    explode_arg = explode.this
    215
    216                    if isinstance(explode, exp.ExplodeOuter):
    217                        bracket = explode_arg[0]
    218                        bracket.set("safe", True)
    219                        bracket.set("offset", True)
    220                        explode_arg = exp.func(
    221                            "IF",
    222                            exp.func(
    223                                "ARRAY_SIZE", exp.func("COALESCE", explode_arg, exp.Array())
    224                            ).eq(0),
    225                            exp.array(bracket, copy=False),
    226                            explode_arg,
    227                        )
    228
    229                    # This ensures that we won't use [POS]EXPLODE's argument as a new selection
    230                    if isinstance(explode_arg, exp.Column):
    231                        taken_select_names.add(explode_arg.output_name)
    232
    233                    unnest_source_alias = new_name(taken_source_names, "_u")
    234
    235                    if not explode_alias:
    236                        explode_alias = new_name(taken_select_names, "col")
    237
    238                        if is_posexplode:
    239                            pos_alias = new_name(taken_select_names, "pos")
    240
    241                    if not pos_alias:
    242                        pos_alias = new_name(taken_select_names, "pos")
    243
    244                    alias.set("alias", exp.to_identifier(explode_alias))
    245
    246                    series_table_alias = series.args["alias"].this
    247                    column = exp.If(
    248                        this=exp.column(series_alias, table=series_table_alias).eq(
    249                            exp.column(pos_alias, table=unnest_source_alias)
    250                        ),
    251                        true=exp.column(explode_alias, table=unnest_source_alias),
    252                    )
    253
    254                    explode.replace(column)
    255
    256                    if is_posexplode:
    257                        expressions = expression.expressions
    258                        expressions.insert(
    259                            expressions.index(alias) + 1,
    260                            exp.If(
    261                                this=exp.column(series_alias, table=series_table_alias).eq(
    262                                    exp.column(pos_alias, table=unnest_source_alias)
    263                                ),
    264                                true=exp.column(pos_alias, table=unnest_source_alias),
    265                            ).as_(pos_alias),
    266                        )
    267                        expression.set("expressions", expressions)
    268
    269                    if not arrays:
    270                        if expression.args.get("from"):
    271                            expression.join(series, copy=False, join_type="CROSS")
    272                        else:
    273                            expression.from_(series, copy=False)
    274
    275                    size: exp.Condition = exp.ArraySize(this=explode_arg.copy())
    276                    arrays.append(size)
    277
    278                    # trino doesn't support left join unnest with on conditions
    279                    # if it did, this would be much simpler
    280                    expression.join(
    281                        exp.alias_(
    282                            exp.Unnest(
    283                                expressions=[explode_arg.copy()],
    284                                offset=exp.to_identifier(pos_alias),
    285                            ),
    286                            unnest_source_alias,
    287                            table=[explode_alias],
    288                        ),
    289                        join_type="CROSS",
    290                        copy=False,
    291                    )
    292
    293                    if index_offset != 1:
    294                        size = size - 1
    295
    296                    expression.where(
    297                        exp.column(series_alias, table=series_table_alias)
    298                        .eq(exp.column(pos_alias, table=unnest_source_alias))
    299                        .or_(
    300                            (exp.column(series_alias, table=series_table_alias) > size).and_(
    301                                exp.column(pos_alias, table=unnest_source_alias).eq(size)
    302                            )
    303                        ),
    304                        copy=False,
    305                    )
    306
    307            if arrays:
    308                end: exp.Condition = exp.Greatest(this=arrays[0], expressions=arrays[1:])
    309
    310                if index_offset != 1:
    311                    end = end - (1 - index_offset)
    312                series.expressions[0].set("end", end)
    313
    314        return expression
    315
    316    return _explode_to_unnest
    317
    318
    319PERCENTILES = (exp.PercentileCont, exp.PercentileDisc)
    320
    321
    322def add_within_group_for_percentiles(expression: exp.Expression) -> exp.Expression:
    323    """Transforms percentiles by adding a WITHIN GROUP clause to them."""
    324    if (
    325        isinstance(expression, PERCENTILES)
    326        and not isinstance(expression.parent, exp.WithinGroup)
    327        and expression.expression
    328    ):
    329        column = expression.this.pop()
    330        expression.set("this", expression.expression.pop())
    331        order = exp.Order(expressions=[exp.Ordered(this=column)])
    332        expression = exp.WithinGroup(this=expression, expression=order)
    333
    334    return expression
    335
    336
    337def remove_within_group_for_percentiles(expression: exp.Expression) -> exp.Expression:
    338    """Transforms percentiles by getting rid of their corresponding WITHIN GROUP clause."""
    339    if (
    340        isinstance(expression, exp.WithinGroup)
    341        and isinstance(expression.this, PERCENTILES)
    342        and isinstance(expression.expression, exp.Order)
    343    ):
    344        quantile = expression.this.this
    345        input_value = t.cast(exp.Ordered, expression.find(exp.Ordered)).this
    346        return expression.replace(exp.ApproxQuantile(this=input_value, quantile=quantile))
    347
    348    return expression
    349
    350
    351def add_recursive_cte_column_names(expression: exp.Expression) -> exp.Expression:
    352    """Uses projection output names in recursive CTE definitions to define the CTEs' columns."""
    353    if isinstance(expression, exp.With) and expression.recursive:
    354        next_name = name_sequence("_c_")
    355
    356        for cte in expression.expressions:
    357            if not cte.args["alias"].columns:
    358                query = cte.this
    359                if isinstance(query, exp.Union):
    360                    query = query.this
    361
    362                cte.args["alias"].set(
    363                    "columns",
    364                    [exp.to_identifier(s.alias_or_name or next_name()) for s in query.selects],
    365                )
    366
    367    return expression
    368
    369
    370def epoch_cast_to_ts(expression: exp.Expression) -> exp.Expression:
    371    """Replace 'epoch' in casts by the equivalent date literal."""
    372    if (
    373        isinstance(expression, (exp.Cast, exp.TryCast))
    374        and expression.name.lower() == "epoch"
    375        and expression.to.this in exp.DataType.TEMPORAL_TYPES
    376    ):
    377        expression.this.replace(exp.Literal.string("1970-01-01 00:00:00"))
    378
    379    return expression
    380
    381
    382def eliminate_semi_and_anti_joins(expression: exp.Expression) -> exp.Expression:
    383    """Convert SEMI and ANTI joins into equivalent forms that use EXIST instead."""
    384    if isinstance(expression, exp.Select):
    385        for join in expression.args.get("joins") or []:
    386            on = join.args.get("on")
    387            if on and join.kind in ("SEMI", "ANTI"):
    388                subquery = exp.select("1").from_(join.this).where(on)
    389                exists = exp.Exists(this=subquery)
    390                if join.kind == "ANTI":
    391                    exists = exists.not_(copy=False)
    392
    393                join.pop()
    394                expression.where(exists, copy=False)
    395
    396    return expression
    397
    398
    399def eliminate_full_outer_join(expression: exp.Expression) -> exp.Expression:
    400    """
    401    Converts a query with a FULL OUTER join to a union of identical queries that
    402    use LEFT/RIGHT OUTER joins instead. This transformation currently only works
    403    for queries that have a single FULL OUTER join.
    404    """
    405    if isinstance(expression, exp.Select):
    406        full_outer_joins = [
    407            (index, join)
    408            for index, join in enumerate(expression.args.get("joins") or [])
    409            if join.side == "FULL"
    410        ]
    411
    412        if len(full_outer_joins) == 1:
    413            expression_copy = expression.copy()
    414            expression.set("limit", None)
    415            index, full_outer_join = full_outer_joins[0]
    416            full_outer_join.set("side", "left")
    417            expression_copy.args["joins"][index].set("side", "right")
    418            expression_copy.args.pop("with", None)  # remove CTEs from RIGHT side
    419
    420            return exp.union(expression, expression_copy, copy=False)
    421
    422    return expression
    423
    424
    425def move_ctes_to_top_level(expression: exp.Expression) -> exp.Expression:
    426    """
    427    Some dialects (e.g. Hive, T-SQL, Spark prior to version 3) only allow CTEs to be
    428    defined at the top-level, so for example queries like:
    429
    430        SELECT * FROM (WITH t(c) AS (SELECT 1) SELECT * FROM t) AS subq
    431
    432    are invalid in those dialects. This transformation can be used to ensure all CTEs are
    433    moved to the top level so that the final SQL code is valid from a syntax standpoint.
    434
    435    TODO: handle name clashes whilst moving CTEs (it can get quite tricky & costly).
    436    """
    437    top_level_with = expression.args.get("with")
    438    for node in expression.find_all(exp.With):
    439        if node.parent is expression:
    440            continue
    441
    442        inner_with = node.pop()
    443        if not top_level_with:
    444            top_level_with = inner_with
    445            expression.set("with", top_level_with)
    446        else:
    447            if inner_with.recursive:
    448                top_level_with.set("recursive", True)
    449
    450            top_level_with.expressions.extend(inner_with.expressions)
    451
    452    return expression
    453
    454
    455def ensure_bools(expression: exp.Expression) -> exp.Expression:
    456    """Converts numeric values used in conditions into explicit boolean expressions."""
    457    from sqlglot.optimizer.canonicalize import ensure_bools
    458
    459    def _ensure_bool(node: exp.Expression) -> None:
    460        if (
    461            node.is_number
    462            or node.is_type(exp.DataType.Type.UNKNOWN, *exp.DataType.NUMERIC_TYPES)
    463            or (isinstance(node, exp.Column) and not node.type)
    464        ):
    465            node.replace(node.neq(0))
    466
    467    for node, *_ in expression.walk():
    468        ensure_bools(node, _ensure_bool)
    469
    470    return expression
    471
    472
    473def unqualify_columns(expression: exp.Expression) -> exp.Expression:
    474    for column in expression.find_all(exp.Column):
    475        # We only wanna pop off the table, db, catalog args
    476        for part in column.parts[:-1]:
    477            part.pop()
    478
    479    return expression
    480
    481
    482def remove_unique_constraints(expression: exp.Expression) -> exp.Expression:
    483    assert isinstance(expression, exp.Create)
    484    for constraint in expression.find_all(exp.UniqueColumnConstraint):
    485        if constraint.parent:
    486            constraint.parent.pop()
    487
    488    return expression
    489
    490
    491def ctas_with_tmp_tables_to_create_tmp_view(
    492    expression: exp.Expression,
    493    tmp_storage_provider: t.Callable[[exp.Expression], exp.Expression] = lambda e: e,
    494) -> exp.Expression:
    495    assert isinstance(expression, exp.Create)
    496    properties = expression.args.get("properties")
    497    temporary = any(
    498        isinstance(prop, exp.TemporaryProperty)
    499        for prop in (properties.expressions if properties else [])
    500    )
    501
    502    # CTAS with temp tables map to CREATE TEMPORARY VIEW
    503    if expression.kind == "TABLE" and temporary:
    504        if expression.expression:
    505            return exp.Create(
    506                kind="TEMPORARY VIEW",
    507                this=expression.this,
    508                expression=expression.expression,
    509            )
    510        return tmp_storage_provider(expression)
    511
    512    return expression
    513
    514
    515def move_schema_columns_to_partitioned_by(expression: exp.Expression) -> exp.Expression:
    516    """
    517    In Hive, the PARTITIONED BY property acts as an extension of a table's schema. When the
    518    PARTITIONED BY value is an array of column names, they are transformed into a schema.
    519    The corresponding columns are removed from the create statement.
    520    """
    521    assert isinstance(expression, exp.Create)
    522    has_schema = isinstance(expression.this, exp.Schema)
    523    is_partitionable = expression.kind in {"TABLE", "VIEW"}
    524
    525    if has_schema and is_partitionable:
    526        prop = expression.find(exp.PartitionedByProperty)
    527        if prop and prop.this and not isinstance(prop.this, exp.Schema):
    528            schema = expression.this
    529            columns = {v.name.upper() for v in prop.this.expressions}
    530            partitions = [col for col in schema.expressions if col.name.upper() in columns]
    531            schema.set("expressions", [e for e in schema.expressions if e not in partitions])
    532            prop.replace(exp.PartitionedByProperty(this=exp.Schema(expressions=partitions)))
    533            expression.set("this", schema)
    534
    535    return expression
    536
    537
    538def move_partitioned_by_to_schema_columns(expression: exp.Expression) -> exp.Expression:
    539    """
    540    Spark 3 supports both "HIVEFORMAT" and "DATASOURCE" formats for CREATE TABLE.
    541
    542    Currently, SQLGlot uses the DATASOURCE format for Spark 3.
    543    """
    544    assert isinstance(expression, exp.Create)
    545    prop = expression.find(exp.PartitionedByProperty)
    546    if (
    547        prop
    548        and prop.this
    549        and isinstance(prop.this, exp.Schema)
    550        and all(isinstance(e, exp.ColumnDef) and e.kind for e in prop.this.expressions)
    551    ):
    552        prop_this = exp.Tuple(
    553            expressions=[exp.to_identifier(e.this) for e in prop.this.expressions]
    554        )
    555        schema = expression.this
    556        for e in prop.this.expressions:
    557            schema.append("expressions", e)
    558        prop.set("this", prop_this)
    559
    560    return expression
    561
    562
    563def struct_kv_to_alias(expression: exp.Expression) -> exp.Expression:
    564    """
    565    Convert struct arguments to aliases: STRUCT(1 AS y) .
    566    """
    567    if isinstance(expression, exp.Struct):
    568        expression.set(
    569            "expressions",
    570            [
    571                exp.alias_(e.expression, e.this) if isinstance(e, exp.PropertyEQ) else e
    572                for e in expression.expressions
    573            ],
    574        )
    575
    576    return expression
    577
    578
    579def preprocess(
    580    transforms: t.List[t.Callable[[exp.Expression], exp.Expression]],
    581) -> t.Callable[[Generator, exp.Expression], str]:
    582    """
    583    Creates a new transform by chaining a sequence of transformations and converts the resulting
    584    expression to SQL, using either the "_sql" method corresponding to the resulting expression,
    585    or the appropriate `Generator.TRANSFORMS` function (when applicable -- see below).
    586
    587    Args:
    588        transforms: sequence of transform functions. These will be called in order.
    589
    590    Returns:
    591        Function that can be used as a generator transform.
    592    """
    593
    594    def _to_sql(self, expression: exp.Expression) -> str:
    595        expression_type = type(expression)
    596
    597        expression = transforms[0](expression)
    598        for transform in transforms[1:]:
    599            expression = transform(expression)
    600
    601        _sql_handler = getattr(self, expression.key + "_sql", None)
    602        if _sql_handler:
    603            return _sql_handler(expression)
    604
    605        transforms_handler = self.TRANSFORMS.get(type(expression))
    606        if transforms_handler:
    607            if expression_type is type(expression):
    608                if isinstance(expression, exp.Func):
    609                    return self.function_fallback_sql(expression)
    610
    611                # Ensures we don't enter an infinite loop. This can happen when the original expression
    612                # has the same type as the final expression and there's no _sql method available for it,
    613                # because then it'd re-enter _to_sql.
    614                raise ValueError(
    615                    f"Expression type {expression.__class__.__name__} requires a _sql method in order to be transformed."
    616                )
    617
    618            return transforms_handler(self, expression)
    619
    620        raise ValueError(f"Unsupported expression type {expression.__class__.__name__}.")
    621
    622    return _to_sql

def unalias_group( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    13def unalias_group(expression: exp.Expression) -> exp.Expression:
    14    """
    15    Replace references to select aliases in GROUP BY clauses.
    16
    17    Example:
    18        >>> import sqlglot
    19        >>> sqlglot.parse_one("SELECT a AS b FROM x GROUP BY b").transform(unalias_group).sql()
    20        'SELECT a AS b FROM x GROUP BY 1'
    21
    22    Args:
    23        expression: the expression that will be transformed.
    24
    25    Returns:
    26        The transformed expression.
    27    """
    28    if isinstance(expression, exp.Group) and isinstance(expression.parent, exp.Select):
    29        aliased_selects = {
    30            e.alias: i
    31            for i, e in enumerate(expression.parent.expressions, start=1)
    32            if isinstance(e, exp.Alias)
    33        }
    34
    35        for group_by in expression.expressions:
    36            if (
    37                isinstance(group_by, exp.Column)
    38                and not group_by.table
    39                and group_by.name in aliased_selects
    40            ):
    41                group_by.replace(exp.Literal.number(aliased_selects.get(group_by.name)))
    42
    43    return expression

Replace references to select aliases in GROUP BY clauses.

###### Example:

>     >>> import sqlglot
>     >>> sqlglot.parse_one("SELECT a AS b FROM x GROUP BY b").transform(unalias_group).sql()
>     'SELECT a AS b FROM x GROUP BY 1'

###### Arguments:

- expression: the expression that will be transformed.

###### Returns:

> The transformed expression.

def eliminate_distinct_on( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    46def eliminate_distinct_on(expression: exp.Expression) -> exp.Expression:
    47    """
    48    Convert SELECT DISTINCT ON statements to a subquery with a window function.
    49
    50    This is useful for dialects that don't support SELECT DISTINCT ON but support window functions.
    51
    52    Args:
    53        expression: the expression that will be transformed.
    54
    55    Returns:
    56        The transformed expression.
    57    """
    58    if (
    59        isinstance(expression, exp.Select)
    60        and expression.args.get("distinct")
    61        and expression.args["distinct"].args.get("on")
    62        and isinstance(expression.args["distinct"].args["on"], exp.Tuple)
    63    ):
    64        distinct_cols = expression.args["distinct"].pop().args["on"].expressions
    65        outer_selects = expression.selects
    66        row_number = find_new_name(expression.named_selects, "_row_number")
    67        window = exp.Window(this=exp.RowNumber(), partition_by=distinct_cols)
    68        order = expression.args.get("order")
    69
    70        if order:
    71            window.set("order", order.pop())
    72        else:
    73            window.set("order", exp.Order(expressions=[c.copy() for c in distinct_cols]))
    74
    75        window = exp.alias_(window, row_number)
    76        expression.select(window, copy=False)
    77
    78        return (
    79            exp.select(*outer_selects, copy=False)
    80            .from_(expression.subquery("_t", copy=False), copy=False)
    81            .where(exp.column(row_number).eq(1), copy=False)
    82        )
    83
    84    return expression

Convert SELECT DISTINCT ON statements to a subquery with a window function.

This is useful for dialects that don't support SELECT DISTINCT ON but support window functions.

###### Arguments:

- expression: the expression that will be transformed.

###### Returns:

> The transformed expression.

def eliminate_qualify( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

     87def eliminate_qualify(expression: exp.Expression) -> exp.Expression:
     88    """
     89    Convert SELECT statements that contain the QUALIFY clause into subqueries, filtered equivalently.
     90
     91    The idea behind this transformation can be seen in Snowflake's documentation for QUALIFY:
     92    https://docs.snowflake.com/en/sql-reference/constructs/qualify
     93
     94    Some dialects don't support window functions in the WHERE clause, so we need to include them as
     95    projections in the subquery, in order to refer to them in the outer filter using aliases. Also,
     96    if a column is referenced in the QUALIFY clause but is not selected, we need to include it too,
     97    otherwise we won't be able to refer to it in the outer query's WHERE clause.
     98    """
     99    if isinstance(expression, exp.Select) and expression.args.get("qualify"):
    100        taken = set(expression.named_selects)
    101        for select in expression.selects:
    102            if not select.alias_or_name:
    103                alias = find_new_name(taken, "_c")
    104                select.replace(exp.alias_(select, alias))
    105                taken.add(alias)
    106
    107        outer_selects = exp.select(*[select.alias_or_name for select in expression.selects])
    108        qualify_filters = expression.args["qualify"].pop().this
    109
    110        select_candidates = exp.Window if expression.is_star else (exp.Window, exp.Column)
    111        for expr in qualify_filters.find_all(select_candidates):
    112            if isinstance(expr, exp.Window):
    113                alias = find_new_name(expression.named_selects, "_w")
    114                expression.select(exp.alias_(expr, alias), copy=False)
    115                column = exp.column(alias)
    116
    117                if isinstance(expr.parent, exp.Qualify):
    118                    qualify_filters = column
    119                else:
    120                    expr.replace(column)
    121            elif expr.name not in expression.named_selects:
    122                expression.select(expr.copy(), copy=False)
    123
    124        return outer_selects.from_(expression.subquery(alias="_t", copy=False), copy=False).where(
    125            qualify_filters, copy=False
    126        )
    127
    128    return expression

Convert SELECT statements that contain the QUALIFY clause into subqueries, filtered equivalently.

The idea behind this transformation can be seen in Snowflake's documentation for QUALIFY: https://docs.snowflake.com/en/sql-reference/constructs/qualify

Some dialects don't support window functions in the WHERE clause, so we need to include them as projections in the subquery, in order to refer to them in the outer filter using aliases. Also, if a column is referenced in the QUALIFY clause but is not selected, we need to include it too, otherwise we won't be able to refer to it in the outer query's WHERE clause.

def remove_precision_parameterized_types( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    131def remove_precision_parameterized_types(expression: exp.Expression) -> exp.Expression:
    132    """
    133    Some dialects only allow the precision for parameterized types to be defined in the DDL and not in
    134    other expressions. This transforms removes the precision from parameterized types in expressions.
    135    """
    136    for node in expression.find_all(exp.DataType):
    137        node.set(
    138            "expressions", [e for e in node.expressions if not isinstance(e, exp.DataTypeParam)]
    139        )
    140
    141    return expression

Some dialects only allow the precision for parameterized types to be defined in the DDL and not in other expressions. This transforms removes the precision from parameterized types in expressions.

def unnest_to_explode( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    144def unnest_to_explode(expression: exp.Expression) -> exp.Expression:
    145    """Convert cross join unnest into lateral view explode."""
    146    if isinstance(expression, exp.Select):
    147        for join in expression.args.get("joins") or []:
    148            unnest = join.this
    149
    150            if isinstance(unnest, exp.Unnest):
    151                alias = unnest.args.get("alias")
    152                udtf = exp.Posexplode if unnest.args.get("offset") else exp.Explode
    153
    154                expression.args["joins"].remove(join)
    155
    156                for e, column in zip(unnest.expressions, alias.columns if alias else []):
    157                    expression.append(
    158                        "laterals",
    159                        exp.Lateral(
    160                            this=udtf(this=e),
    161                            view=True,
    162                            alias=exp.TableAlias(this=alias.this, columns=[column]),  # type: ignore
    163                        ),
    164                    )
    165
    166    return expression

Convert cross join unnest into lateral view explode.

def explode_to_unnest( index_offset: int = 0) -> Callable[[sqlglot.expressions.Expression], sqlglot.expressions.Expression]: View Source

    169def explode_to_unnest(index_offset: int = 0) -> t.Callable[[exp.Expression], exp.Expression]:
    170    """Convert explode/posexplode into unnest."""
    171
    172    def _explode_to_unnest(expression: exp.Expression) -> exp.Expression:
    173        if isinstance(expression, exp.Select):
    174            from sqlglot.optimizer.scope import Scope
    175
    176            taken_select_names = set(expression.named_selects)
    177            taken_source_names = {name for name, _ in Scope(expression).references}
    178
    179            def new_name(names: t.Set[str], name: str) -> str:
    180                name = find_new_name(names, name)
    181                names.add(name)
    182                return name
    183
    184            arrays: t.List[exp.Condition] = []
    185            series_alias = new_name(taken_select_names, "pos")
    186            series = exp.alias_(
    187                exp.Unnest(
    188                    expressions=[exp.GenerateSeries(start=exp.Literal.number(index_offset))]
    189                ),
    190                new_name(taken_source_names, "_u"),
    191                table=[series_alias],
    192            )
    193
    194            # we use list here because expression.selects is mutated inside the loop
    195            for select in list(expression.selects):
    196                explode = select.find(exp.Explode)
    197
    198                if explode:
    199                    pos_alias = ""
    200                    explode_alias = ""
    201
    202                    if isinstance(select, exp.Alias):
    203                        explode_alias = select.args["alias"]
    204                        alias = select
    205                    elif isinstance(select, exp.Aliases):
    206                        pos_alias = select.aliases[0]
    207                        explode_alias = select.aliases[1]
    208                        alias = select.replace(exp.alias_(select.this, "", copy=False))
    209                    else:
    210                        alias = select.replace(exp.alias_(select, ""))
    211                        explode = alias.find(exp.Explode)
    212                        assert explode
    213
    214                    is_posexplode = isinstance(explode, exp.Posexplode)
    215                    explode_arg = explode.this
    216
    217                    if isinstance(explode, exp.ExplodeOuter):
    218                        bracket = explode_arg[0]
    219                        bracket.set("safe", True)
    220                        bracket.set("offset", True)
    221                        explode_arg = exp.func(
    222                            "IF",
    223                            exp.func(
    224                                "ARRAY_SIZE", exp.func("COALESCE", explode_arg, exp.Array())
    225                            ).eq(0),
    226                            exp.array(bracket, copy=False),
    227                            explode_arg,
    228                        )
    229
    230                    # This ensures that we won't use [POS]EXPLODE's argument as a new selection
    231                    if isinstance(explode_arg, exp.Column):
    232                        taken_select_names.add(explode_arg.output_name)
    233
    234                    unnest_source_alias = new_name(taken_source_names, "_u")
    235
    236                    if not explode_alias:
    237                        explode_alias = new_name(taken_select_names, "col")
    238
    239                        if is_posexplode:
    240                            pos_alias = new_name(taken_select_names, "pos")
    241
    242                    if not pos_alias:
    243                        pos_alias = new_name(taken_select_names, "pos")
    244
    245                    alias.set("alias", exp.to_identifier(explode_alias))
    246
    247                    series_table_alias = series.args["alias"].this
    248                    column = exp.If(
    249                        this=exp.column(series_alias, table=series_table_alias).eq(
    250                            exp.column(pos_alias, table=unnest_source_alias)
    251                        ),
    252                        true=exp.column(explode_alias, table=unnest_source_alias),
    253                    )
    254
    255                    explode.replace(column)
    256
    257                    if is_posexplode:
    258                        expressions = expression.expressions
    259                        expressions.insert(
    260                            expressions.index(alias) + 1,
    261                            exp.If(
    262                                this=exp.column(series_alias, table=series_table_alias).eq(
    263                                    exp.column(pos_alias, table=unnest_source_alias)
    264                                ),
    265                                true=exp.column(pos_alias, table=unnest_source_alias),
    266                            ).as_(pos_alias),
    267                        )
    268                        expression.set("expressions", expressions)
    269
    270                    if not arrays:
    271                        if expression.args.get("from"):
    272                            expression.join(series, copy=False, join_type="CROSS")
    273                        else:
    274                            expression.from_(series, copy=False)
    275
    276                    size: exp.Condition = exp.ArraySize(this=explode_arg.copy())
    277                    arrays.append(size)
    278
    279                    # trino doesn't support left join unnest with on conditions
    280                    # if it did, this would be much simpler
    281                    expression.join(
    282                        exp.alias_(
    283                            exp.Unnest(
    284                                expressions=[explode_arg.copy()],
    285                                offset=exp.to_identifier(pos_alias),
    286                            ),
    287                            unnest_source_alias,
    288                            table=[explode_alias],
    289                        ),
    290                        join_type="CROSS",
    291                        copy=False,
    292                    )
    293
    294                    if index_offset != 1:
    295                        size = size - 1
    296
    297                    expression.where(
    298                        exp.column(series_alias, table=series_table_alias)
    299                        .eq(exp.column(pos_alias, table=unnest_source_alias))
    300                        .or_(
    301                            (exp.column(series_alias, table=series_table_alias) > size).and_(
    302                                exp.column(pos_alias, table=unnest_source_alias).eq(size)
    303                            )
    304                        ),
    305                        copy=False,
    306                    )
    307
    308            if arrays:
    309                end: exp.Condition = exp.Greatest(this=arrays[0], expressions=arrays[1:])
    310
    311                if index_offset != 1:
    312                    end = end - (1 - index_offset)
    313                series.expressions[0].set("end", end)
    314
    315        return expression
    316
    317    return _explode_to_unnest

Convert explode/posexplode into unnest.

PERCENTILES = (<class 'sqlglot.expressions.PercentileCont'>, <class 'sqlglot.expressions.PercentileDisc'>)
def add_within_group_for_percentiles( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    323def add_within_group_for_percentiles(expression: exp.Expression) -> exp.Expression:
    324    """Transforms percentiles by adding a WITHIN GROUP clause to them."""
    325    if (
    326        isinstance(expression, PERCENTILES)
    327        and not isinstance(expression.parent, exp.WithinGroup)
    328        and expression.expression
    329    ):
    330        column = expression.this.pop()
    331        expression.set("this", expression.expression.pop())
    332        order = exp.Order(expressions=[exp.Ordered(this=column)])
    333        expression = exp.WithinGroup(this=expression, expression=order)
    334
    335    return expression

Transforms percentiles by adding a WITHIN GROUP clause to them.

def remove_within_group_for_percentiles( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    338def remove_within_group_for_percentiles(expression: exp.Expression) -> exp.Expression:
    339    """Transforms percentiles by getting rid of their corresponding WITHIN GROUP clause."""
    340    if (
    341        isinstance(expression, exp.WithinGroup)
    342        and isinstance(expression.this, PERCENTILES)
    343        and isinstance(expression.expression, exp.Order)
    344    ):
    345        quantile = expression.this.this
    346        input_value = t.cast(exp.Ordered, expression.find(exp.Ordered)).this
    347        return expression.replace(exp.ApproxQuantile(this=input_value, quantile=quantile))
    348
    349    return expression

Transforms percentiles by getting rid of their corresponding WITHIN GROUP clause.

def add_recursive_cte_column_names( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    352def add_recursive_cte_column_names(expression: exp.Expression) -> exp.Expression:
    353    """Uses projection output names in recursive CTE definitions to define the CTEs' columns."""
    354    if isinstance(expression, exp.With) and expression.recursive:
    355        next_name = name_sequence("_c_")
    356
    357        for cte in expression.expressions:
    358            if not cte.args["alias"].columns:
    359                query = cte.this
    360                if isinstance(query, exp.Union):
    361                    query = query.this
    362
    363                cte.args["alias"].set(
    364                    "columns",
    365                    [exp.to_identifier(s.alias_or_name or next_name()) for s in query.selects],
    366                )
    367
    368    return expression

Uses projection output names in recursive CTE definitions to define the CTEs' columns.

def epoch_cast_to_ts( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    371def epoch_cast_to_ts(expression: exp.Expression) -> exp.Expression:
    372    """Replace 'epoch' in casts by the equivalent date literal."""
    373    if (
    374        isinstance(expression, (exp.Cast, exp.TryCast))
    375        and expression.name.lower() == "epoch"
    376        and expression.to.this in exp.DataType.TEMPORAL_TYPES
    377    ):
    378        expression.this.replace(exp.Literal.string("1970-01-01 00:00:00"))
    379
    380    return expression

Replace 'epoch' in casts by the equivalent date literal.

def eliminate_semi_and_anti_joins( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    383def eliminate_semi_and_anti_joins(expression: exp.Expression) -> exp.Expression:
    384    """Convert SEMI and ANTI joins into equivalent forms that use EXIST instead."""
    385    if isinstance(expression, exp.Select):
    386        for join in expression.args.get("joins") or []:
    387            on = join.args.get("on")
    388            if on and join.kind in ("SEMI", "ANTI"):
    389                subquery = exp.select("1").from_(join.this).where(on)
    390                exists = exp.Exists(this=subquery)
    391                if join.kind == "ANTI":
    392                    exists = exists.not_(copy=False)
    393
    394                join.pop()
    395                expression.where(exists, copy=False)
    396
    397    return expression

Convert SEMI and ANTI joins into equivalent forms that use EXIST instead.

def eliminate_full_outer_join( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    400def eliminate_full_outer_join(expression: exp.Expression) -> exp.Expression:
    401    """
    402    Converts a query with a FULL OUTER join to a union of identical queries that
    403    use LEFT/RIGHT OUTER joins instead. This transformation currently only works
    404    for queries that have a single FULL OUTER join.
    405    """
    406    if isinstance(expression, exp.Select):
    407        full_outer_joins = [
    408            (index, join)
    409            for index, join in enumerate(expression.args.get("joins") or [])
    410            if join.side == "FULL"
    411        ]
    412
    413        if len(full_outer_joins) == 1:
    414            expression_copy = expression.copy()
    415            expression.set("limit", None)
    416            index, full_outer_join = full_outer_joins[0]
    417            full_outer_join.set("side", "left")
    418            expression_copy.args["joins"][index].set("side", "right")
    419            expression_copy.args.pop("with", None)  # remove CTEs from RIGHT side
    420
    421            return exp.union(expression, expression_copy, copy=False)
    422
    423    return expression

Converts a query with a FULL OUTER join to a union of identical queries that use LEFT/RIGHT OUTER joins instead. This transformation currently only works for queries that have a single FULL OUTER join.

def move_ctes_to_top_level( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    426def move_ctes_to_top_level(expression: exp.Expression) -> exp.Expression:
    427    """
    428    Some dialects (e.g. Hive, T-SQL, Spark prior to version 3) only allow CTEs to be
    429    defined at the top-level, so for example queries like:
    430
    431        SELECT * FROM (WITH t(c) AS (SELECT 1) SELECT * FROM t) AS subq
    432
    433    are invalid in those dialects. This transformation can be used to ensure all CTEs are
    434    moved to the top level so that the final SQL code is valid from a syntax standpoint.
    435
    436    TODO: handle name clashes whilst moving CTEs (it can get quite tricky & costly).
    437    """
    438    top_level_with = expression.args.get("with")
    439    for node in expression.find_all(exp.With):
    440        if node.parent is expression:
    441            continue
    442
    443        inner_with = node.pop()
    444        if not top_level_with:
    445            top_level_with = inner_with
    446            expression.set("with", top_level_with)
    447        else:
    448            if inner_with.recursive:
    449                top_level_with.set("recursive", True)
    450
    451            top_level_with.expressions.extend(inner_with.expressions)
    452
    453    return expression

Some dialects (e.g. Hive, T-SQL, Spark prior to version 3) only allow CTEs to be defined at the top-level, so for example queries like:

    SELECT * FROM (WITH t(c) AS (SELECT 1) SELECT * FROM t) AS subq

are invalid in those dialects. This transformation can be used to ensure all CTEs are moved to the top level so that the final SQL code is valid from a syntax standpoint.

TODO: handle name clashes whilst moving CTEs (it can get quite tricky & costly).

def ensure_bools( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    456def ensure_bools(expression: exp.Expression) -> exp.Expression:
    457    """Converts numeric values used in conditions into explicit boolean expressions."""
    458    from sqlglot.optimizer.canonicalize import ensure_bools
    459
    460    def _ensure_bool(node: exp.Expression) -> None:
    461        if (
    462            node.is_number
    463            or node.is_type(exp.DataType.Type.UNKNOWN, *exp.DataType.NUMERIC_TYPES)
    464            or (isinstance(node, exp.Column) and not node.type)
    465        ):
    466            node.replace(node.neq(0))
    467
    468    for node, *_ in expression.walk():
    469        ensure_bools(node, _ensure_bool)
    470
    471    return expression

Converts numeric values used in conditions into explicit boolean expressions.

def unqualify_columns( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    474def unqualify_columns(expression: exp.Expression) -> exp.Expression:
    475    for column in expression.find_all(exp.Column):
    476        # We only wanna pop off the table, db, catalog args
    477        for part in column.parts[:-1]:
    478            part.pop()
    479
    480    return expression

def remove_unique_constraints( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    483def remove_unique_constraints(expression: exp.Expression) -> exp.Expression:
    484    assert isinstance(expression, exp.Create)
    485    for constraint in expression.find_all(exp.UniqueColumnConstraint):
    486        if constraint.parent:
    487            constraint.parent.pop()
    488
    489    return expression

def ctas_with_tmp_tables_to_create_tmp_view( expression: sqlglot.expressions.Expression, tmp_storage_provider: Callable[[sqlglot.expressions.Expression], sqlglot.expressions.Expression] = <function <lambda>>) -> sqlglot.expressions.Expression: View Source

    492def ctas_with_tmp_tables_to_create_tmp_view(
    493    expression: exp.Expression,
    494    tmp_storage_provider: t.Callable[[exp.Expression], exp.Expression] = lambda e: e,
    495) -> exp.Expression:
    496    assert isinstance(expression, exp.Create)
    497    properties = expression.args.get("properties")
    498    temporary = any(
    499        isinstance(prop, exp.TemporaryProperty)
    500        for prop in (properties.expressions if properties else [])
    501    )
    502
    503    # CTAS with temp tables map to CREATE TEMPORARY VIEW
    504    if expression.kind == "TABLE" and temporary:
    505        if expression.expression:
    506            return exp.Create(
    507                kind="TEMPORARY VIEW",
    508                this=expression.this,
    509                expression=expression.expression,
    510            )
    511        return tmp_storage_provider(expression)
    512
    513    return expression

def move_schema_columns_to_partitioned_by( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    516def move_schema_columns_to_partitioned_by(expression: exp.Expression) -> exp.Expression:
    517    """
    518    In Hive, the PARTITIONED BY property acts as an extension of a table's schema. When the
    519    PARTITIONED BY value is an array of column names, they are transformed into a schema.
    520    The corresponding columns are removed from the create statement.
    521    """
    522    assert isinstance(expression, exp.Create)
    523    has_schema = isinstance(expression.this, exp.Schema)
    524    is_partitionable = expression.kind in {"TABLE", "VIEW"}
    525
    526    if has_schema and is_partitionable:
    527        prop = expression.find(exp.PartitionedByProperty)
    528        if prop and prop.this and not isinstance(prop.this, exp.Schema):
    529            schema = expression.this
    530            columns = {v.name.upper() for v in prop.this.expressions}
    531            partitions = [col for col in schema.expressions if col.name.upper() in columns]
    532            schema.set("expressions", [e for e in schema.expressions if e not in partitions])
    533            prop.replace(exp.PartitionedByProperty(this=exp.Schema(expressions=partitions)))
    534            expression.set("this", schema)
    535
    536    return expression

In Hive, the PARTITIONED BY property acts as an extension of a table's schema. When the PARTITIONED BY value is an array of column names, they are transformed into a schema. The corresponding columns are removed from the create statement.

def move_partitioned_by_to_schema_columns( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    539def move_partitioned_by_to_schema_columns(expression: exp.Expression) -> exp.Expression:
    540    """
    541    Spark 3 supports both "HIVEFORMAT" and "DATASOURCE" formats for CREATE TABLE.
    542
    543    Currently, SQLGlot uses the DATASOURCE format for Spark 3.
    544    """
    545    assert isinstance(expression, exp.Create)
    546    prop = expression.find(exp.PartitionedByProperty)
    547    if (
    548        prop
    549        and prop.this
    550        and isinstance(prop.this, exp.Schema)
    551        and all(isinstance(e, exp.ColumnDef) and e.kind for e in prop.this.expressions)
    552    ):
    553        prop_this = exp.Tuple(
    554            expressions=[exp.to_identifier(e.this) for e in prop.this.expressions]
    555        )
    556        schema = expression.this
    557        for e in prop.this.expressions:
    558            schema.append("expressions", e)
    559        prop.set("this", prop_this)
    560
    561    return expression

Spark 3 supports both "HIVEFORMAT" and "DATASOURCE" formats for CREATE TABLE.

Currently, SQLGlot uses the DATASOURCE format for Spark 3.

def struct_kv_to_alias( expression: sqlglot.expressions.Expression) -> sqlglot.expressions.Expression: View Source

    564def struct_kv_to_alias(expression: exp.Expression) -> exp.Expression:
    565    """
    566    Convert struct arguments to aliases: STRUCT(1 AS y) .
    567    """
    568    if isinstance(expression, exp.Struct):
    569        expression.set(
    570            "expressions",
    571            [
    572                exp.alias_(e.expression, e.this) if isinstance(e, exp.PropertyEQ) else e
    573                for e in expression.expressions
    574            ],
    575        )
    576
    577    return expression

Convert struct arguments to aliases: STRUCT(1 AS y) .

def preprocess( transforms: List[Callable[[sqlglot.expressions.Expression], sqlglot.expressions.Expression]]) -> Callable[[sqlglot.generator.Generator, sqlglot.expressions.Expression], str]: View Source

    580def preprocess(
    581    transforms: t.List[t.Callable[[exp.Expression], exp.Expression]],
    582) -> t.Callable[[Generator, exp.Expression], str]:
    583    """
    584    Creates a new transform by chaining a sequence of transformations and converts the resulting
    585    expression to SQL, using either the "_sql" method corresponding to the resulting expression,
    586    or the appropriate `Generator.TRANSFORMS` function (when applicable -- see below).
    587
    588    Args:
    589        transforms: sequence of transform functions. These will be called in order.
    590
    591    Returns:
    592        Function that can be used as a generator transform.
    593    """
    594
    595    def _to_sql(self, expression: exp.Expression) -> str:
    596        expression_type = type(expression)
    597
    598        expression = transforms[0](expression)
    599        for transform in transforms[1:]:
    600            expression = transform(expression)
    601
    602        _sql_handler = getattr(self, expression.key + "_sql", None)
    603        if _sql_handler:
    604            return _sql_handler(expression)
    605
    606        transforms_handler = self.TRANSFORMS.get(type(expression))
    607        if transforms_handler:
    608            if expression_type is type(expression):
    609                if isinstance(expression, exp.Func):
    610                    return self.function_fallback_sql(expression)
    611
    612                # Ensures we don't enter an infinite loop. This can happen when the original expression
    613                # has the same type as the final expression and there's no _sql method available for it,
    614                # because then it'd re-enter _to_sql.
    615                raise ValueError(
    616                    f"Expression type {expression.__class__.__name__} requires a _sql method in order to be transformed."
    617                )
    618
    619            return transforms_handler(self, expression)
    620
    621        raise ValueError(f"Unsupported expression type {expression.__class__.__name__}.")
    622
    623    return _to_sql

Creates a new transform by chaining a sequence of transformations and converts the resulting expression to SQL, using either the "_sql" method corresponding to the resulting expression, or the appropriate `Generator.TRANSFORMS` function (when applicable -- see below).

###### Arguments:

- transforms: sequence of transform functions. These will be called in order.

###### Returns:

> Function that can be used as a generator transform.