sqlglot.optimizer.eliminate_subqueries API documentation

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# sqlglot.optimizer.eliminate_subqueries

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      1from __future__ import annotations
      2
      3import itertools
      4import typing as t
      5
      6from sqlglot import expressions as exp
      7from sqlglot.helper import find_new_name
      8from sqlglot.optimizer.scope import Scope, build_scope
      9
     10if t.TYPE_CHECKING:
     11    ExistingCTEsMapping = t.Dict[exp.Expression, str]
     12    TakenNameMapping = t.Dict[str, t.Union[Scope, exp.Expression]]
     13
     14
     15def eliminate_subqueries(expression: exp.Expression) -> exp.Expression:
     16    """
     17    Rewrite derived tables as CTES, deduplicating if possible.
     18
     19    Example:
     20        >>> import sqlglot
     21        >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT * FROM x) AS y")
     22        >>> eliminate_subqueries(expression).sql()
     23        'WITH y AS (SELECT * FROM x) SELECT a FROM y AS y'
     24
     25    This also deduplicates common subqueries:
     26        >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT * FROM x) AS y CROSS JOIN (SELECT * FROM x) AS z")
     27        >>> eliminate_subqueries(expression).sql()
     28        'WITH y AS (SELECT * FROM x) SELECT a FROM y AS y CROSS JOIN y AS z'
     29
     30    Args:
     31        expression (sqlglot.Expression): expression
     32    Returns:
     33        sqlglot.Expression: expression
     34    """
     35    if isinstance(expression, exp.Subquery):
     36        # It's possible to have subqueries at the root, e.g. (SELECT * FROM x) LIMIT 1
     37        eliminate_subqueries(expression.this)
     38        return expression
     39
     40    root = build_scope(expression)
     41
     42    if not root:
     43        return expression
     44
     45    # Map of alias->Scope|Table
     46    # These are all aliases that are already used in the expression.
     47    # We don't want to create new CTEs that conflict with these names.
     48    taken: TakenNameMapping = {}
     49
     50    # All CTE aliases in the root scope are taken
     51    for scope in root.cte_scopes:
     52        taken[scope.expression.parent.alias] = scope
     53
     54    # All table names are taken
     55    for scope in root.traverse():
     56        taken.update(
     57            {
     58                source.name: source
     59                for _, source in scope.sources.items()
     60                if isinstance(source, exp.Table)
     61            }
     62        )
     63
     64    # Map of Expression->alias
     65    # Existing CTES in the root expression. We'll use this for deduplication.
     66    existing_ctes: ExistingCTEsMapping = {}
     67
     68    with_ = root.expression.args.get("with")
     69    recursive = False
     70    if with_:
     71        recursive = with_.args.get("recursive")
     72        for cte in with_.expressions:
     73            existing_ctes[cte.this] = cte.alias
     74    new_ctes = []
     75
     76    # We're adding more CTEs, but we want to maintain the DAG order.
     77    # Derived tables within an existing CTE need to come before the existing CTE.
     78    for cte_scope in root.cte_scopes:
     79        # Append all the new CTEs from this existing CTE
     80        for scope in cte_scope.traverse():
     81            if scope is cte_scope:
     82                # Don't try to eliminate this CTE itself
     83                continue
     84            new_cte = _eliminate(scope, existing_ctes, taken)
     85            if new_cte:
     86                new_ctes.append(new_cte)
     87
     88        # Append the existing CTE itself
     89        new_ctes.append(cte_scope.expression.parent)
     90
     91    # Now append the rest
     92    for scope in itertools.chain(root.union_scopes, root.subquery_scopes, root.table_scopes):
     93        for child_scope in scope.traverse():
     94            new_cte = _eliminate(child_scope, existing_ctes, taken)
     95            if new_cte:
     96                new_ctes.append(new_cte)
     97
     98    if new_ctes:
     99        query = expression.expression if isinstance(expression, exp.DDL) else expression
    100        query.set("with", exp.With(expressions=new_ctes, recursive=recursive))
    101
    102    return expression
    103
    104
    105def _eliminate(
    106    scope: Scope, existing_ctes: ExistingCTEsMapping, taken: TakenNameMapping
    107) -> t.Optional[exp.Expression]:
    108    if scope.is_derived_table:
    109        return _eliminate_derived_table(scope, existing_ctes, taken)
    110
    111    if scope.is_cte:
    112        return _eliminate_cte(scope, existing_ctes, taken)
    113
    114    return None
    115
    116
    117def _eliminate_derived_table(
    118    scope: Scope, existing_ctes: ExistingCTEsMapping, taken: TakenNameMapping
    119) -> t.Optional[exp.Expression]:
    120    # This makes sure that we don't:
    121    # - drop the "pivot" arg from a pivoted subquery
    122    # - eliminate a lateral correlated subquery
    123    if scope.parent.pivots or isinstance(scope.parent.expression, exp.Lateral):
    124        return None
    125
    126    # Get rid of redundant exp.Subquery expressions, i.e. those that are just used as wrappers
    127    to_replace = scope.expression.parent.unwrap()
    128    name, cte = _new_cte(scope, existing_ctes, taken)
    129    table = exp.alias_(exp.table_(name), alias=to_replace.alias or name)
    130    table.set("joins", to_replace.args.get("joins"))
    131
    132    to_replace.replace(table)
    133
    134    return cte
    135
    136
    137def _eliminate_cte(
    138    scope: Scope, existing_ctes: ExistingCTEsMapping, taken: TakenNameMapping
    139) -> t.Optional[exp.Expression]:
    140    parent = scope.expression.parent
    141    name, cte = _new_cte(scope, existing_ctes, taken)
    142
    143    with_ = parent.parent
    144    parent.pop()
    145    if not with_.expressions:
    146        with_.pop()
    147
    148    # Rename references to this CTE
    149    for child_scope in scope.parent.traverse():
    150        for table, source in child_scope.selected_sources.values():
    151            if source is scope:
    152                new_table = exp.alias_(exp.table_(name), alias=table.alias_or_name, copy=False)
    153                table.replace(new_table)
    154
    155    return cte
    156
    157
    158def _new_cte(
    159    scope: Scope, existing_ctes: ExistingCTEsMapping, taken: TakenNameMapping
    160) -> t.Tuple[str, t.Optional[exp.Expression]]:
    161    """
    162    Returns:
    163        tuple of (name, cte)
    164        where `name` is a new name for this CTE in the root scope and `cte` is a new CTE instance.
    165        If this CTE duplicates an existing CTE, `cte` will be None.
    166    """
    167    duplicate_cte_alias = existing_ctes.get(scope.expression)
    168    parent = scope.expression.parent
    169    name = parent.alias
    170
    171    if not name:
    172        name = find_new_name(taken=taken, base="cte")
    173
    174    if duplicate_cte_alias:
    175        name = duplicate_cte_alias
    176    elif taken.get(name):
    177        name = find_new_name(taken=taken, base=name)
    178
    179    taken[name] = scope
    180
    181    if not duplicate_cte_alias:
    182        existing_ctes[scope.expression] = name
    183        cte = exp.CTE(
    184            this=scope.expression,
    185            alias=exp.TableAlias(this=exp.to_identifier(name)),
    186        )
    187    else:
    188        cte = None
    189    return name, cte

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

     16def eliminate_subqueries(expression: exp.Expression) -> exp.Expression:
     17    """
     18    Rewrite derived tables as CTES, deduplicating if possible.
     19
     20    Example:
     21        >>> import sqlglot
     22        >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT * FROM x) AS y")
     23        >>> eliminate_subqueries(expression).sql()
     24        'WITH y AS (SELECT * FROM x) SELECT a FROM y AS y'
     25
     26    This also deduplicates common subqueries:
     27        >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT * FROM x) AS y CROSS JOIN (SELECT * FROM x) AS z")
     28        >>> eliminate_subqueries(expression).sql()
     29        'WITH y AS (SELECT * FROM x) SELECT a FROM y AS y CROSS JOIN y AS z'
     30
     31    Args:
     32        expression (sqlglot.Expression): expression
     33    Returns:
     34        sqlglot.Expression: expression
     35    """
     36    if isinstance(expression, exp.Subquery):
     37        # It's possible to have subqueries at the root, e.g. (SELECT * FROM x) LIMIT 1
     38        eliminate_subqueries(expression.this)
     39        return expression
     40
     41    root = build_scope(expression)
     42
     43    if not root:
     44        return expression
     45
     46    # Map of alias->Scope|Table
     47    # These are all aliases that are already used in the expression.
     48    # We don't want to create new CTEs that conflict with these names.
     49    taken: TakenNameMapping = {}
     50
     51    # All CTE aliases in the root scope are taken
     52    for scope in root.cte_scopes:
     53        taken[scope.expression.parent.alias] = scope
     54
     55    # All table names are taken
     56    for scope in root.traverse():
     57        taken.update(
     58            {
     59                source.name: source
     60                for _, source in scope.sources.items()
     61                if isinstance(source, exp.Table)
     62            }
     63        )
     64
     65    # Map of Expression->alias
     66    # Existing CTES in the root expression. We'll use this for deduplication.
     67    existing_ctes: ExistingCTEsMapping = {}
     68
     69    with_ = root.expression.args.get("with")
     70    recursive = False
     71    if with_:
     72        recursive = with_.args.get("recursive")
     73        for cte in with_.expressions:
     74            existing_ctes[cte.this] = cte.alias
     75    new_ctes = []
     76
     77    # We're adding more CTEs, but we want to maintain the DAG order.
     78    # Derived tables within an existing CTE need to come before the existing CTE.
     79    for cte_scope in root.cte_scopes:
     80        # Append all the new CTEs from this existing CTE
     81        for scope in cte_scope.traverse():
     82            if scope is cte_scope:
     83                # Don't try to eliminate this CTE itself
     84                continue
     85            new_cte = _eliminate(scope, existing_ctes, taken)
     86            if new_cte:
     87                new_ctes.append(new_cte)
     88
     89        # Append the existing CTE itself
     90        new_ctes.append(cte_scope.expression.parent)
     91
     92    # Now append the rest
     93    for scope in itertools.chain(root.union_scopes, root.subquery_scopes, root.table_scopes):
     94        for child_scope in scope.traverse():
     95            new_cte = _eliminate(child_scope, existing_ctes, taken)
     96            if new_cte:
     97                new_ctes.append(new_cte)
     98
     99    if new_ctes:
    100        query = expression.expression if isinstance(expression, exp.DDL) else expression
    101        query.set("with", exp.With(expressions=new_ctes, recursive=recursive))
    102
    103    return expression

Rewrite derived tables as CTES, deduplicating if possible.

###### Example:

>     >>> import sqlglot
>     >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT * FROM x) AS y")
>     >>> eliminate_subqueries(expression).sql()
>     'WITH y AS (SELECT * FROM x) SELECT a FROM y AS y'

###### This also deduplicates common subqueries:

>     >>> expression = sqlglot.parse_one("SELECT a FROM (SELECT * FROM x) AS y CROSS JOIN (SELECT * FROM x) AS z")
>     >>> eliminate_subqueries(expression).sql()
>     'WITH y AS (SELECT * FROM x) SELECT a FROM y AS y CROSS JOIN y AS z'

###### Arguments:

- expression (sqlglot.Expression): expression

###### Returns:

> sqlglot.Expression: expression