Database Access from PL/Tcl

Postgres-XC 1.1 Documentation
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# 41.5. Database Access from PL/Tcl

> Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.

The following commands are available to access the database from the body of a PL/Tcl function:

``spi_exec` ?-count `n`? ?-array `name`? `command` ?`loop-body`?`

  Executes an SQL command given as a string. An error in the command causes an error to be raised. Otherwise, the return value of `spi_exec` is the number of rows processed (selected, inserted, updated, or deleted) by the command, or zero if the command is a utility statement. In addition, if the command is a `SELECT` statement, the values of the selected columns are placed in Tcl variables as described below.

  The optional `-count` value tells `spi_exec` the maximum number of rows to process in the command. The effect of this is comparable to setting up a query as a cursor and then saying `FETCH `n``.

  If the command is a `SELECT` statement, the values of the result columns are placed into Tcl variables named after the columns. If the `-array` option is given, the column values are instead stored into the named associative array, with the column names used as array indexes.

  If the command is a `SELECT` statement and no `loop-body` script is given, then only the first row of results are stored into Tcl variables; remaining rows, if any, are ignored. No storing occurs if the query returns no rows. (This case can be detected by checking the result of `spi_exec`.) For example:

      spi_exec "SELECT count(*) AS cnt FROM pg_proc"

  will set the Tcl variable `$cnt` to the number of rows in the `pg_proc` system catalog.

  If the optional `loop-body` argument is given, it is a piece of Tcl script that is executed once for each row in the query result. (`loop-body` is ignored if the given command is not a `SELECT`.) The values of the current row's columns are stored into Tcl variables before each iteration. For example:

      spi_exec -array C "SELECT * FROM pg_class" {
          elog DEBUG "have table $C(relname)"
      }

  will print a log message for every row of `pg_class`. This feature works similarly to other Tcl looping constructs; in particular `continue` and `break` work in the usual way inside the loop body.

  If a column of a query result is null, the target variable for it is "unset" rather than being set.

`spi_prepare` `query` `typelist`

  Postgres-XC does not support `PREPARE` reature yet. This will be supported in the future releases.

``spi_execp` ?-count `n`? ?-array `name`? ?-nulls `string`? `queryid` ?`value-list`? ?`loop-body`?`

  > Note: The following description applies only to Postgres-XC

  Postgres-XC does not support `PREPARE` in the current release. This may be supported in the future releases.

`spi_lastoid`

  Returns the OID of the row inserted by the last `spi_exec` or `spi_execp`, if the command was a single-row `INSERT` and the modified table contained OIDs. (If not, you get zero.)

  > Note: The following description applies only to Postgres-XC

  Please note that OID is maintained locally at each Datanode and Coordinator.

`quote` `string`

  Doubles all occurrences of single quote and backslash characters in the given string. This can be used to safely quote strings that are to be inserted into SQL commands given to `spi_exec` or `spi_prepare`. For example, think about an SQL command string like:

      "SELECT '$val' AS ret"

  where the Tcl variable `val` actually contains `doesn't`. This would result in the final command string:

      SELECT 'doesn't' AS ret

  which would cause a parse error during `spi_exec` or `spi_prepare`. To work properly, the submitted command should contain:

      SELECT 'doesn''t' AS ret

  which can be formed in PL/Tcl using:

      "SELECT '[ quote $val ]' AS ret"

  One advantage of `spi_execp` is that you don't have to quote parameter values like this, since the parameters are never parsed as part of an SQL command string.

`elog` `level` `msg`

  Emits a log or error message. Possible levels are `DEBUG`, `LOG`, `INFO`, `NOTICE`, `WARNING`, `ERROR`, and `FATAL`. `ERROR` raises an error condition; if this is not trapped by the surrounding Tcl code, the error propagates out to the calling query, causing the current transaction or subtransaction to be aborted. This is effectively the same as the Tcl `error` command. `FATAL` aborts the transaction and causes the current session to shut down. (There is probably no good reason to use this error level in PL/Tcl functions, but it's provided for completeness.) The other levels only generate messages of different priority levels. Whether messages of a particular priority are reported to the client, written to the server log, or both is controlled by the log_min_messages and client_min_messages configuration variables. See Chapter 17 for more information.

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