PostgreSQL 7.4beta4 Documentation
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# 40.5. Database Access from PL/Tcl

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

  Prepares and saves a query plan for later execution. The saved plan will be retained for the life of the current session.

  The query may use parameters, that is, placeholders for values to be supplied whenever the plan is actually executed. In the query string, refer to parameters by the symbols $1 ... $n. If the query uses parameters, the names of the parameter types must be given as a Tcl list. (Write an empty list for typelist if no parameters are used.) Presently, the parameter types must be identified by the internal type names shown in the system table pg_type; for example int4 not integer.

  The return value from `spi_prepare` is a query ID to be used in subsequent calls to `spi_execp`. See `spi_execp` for an example.

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

  Executes a query previously prepared with `spi_prepare`. queryid is the ID returned by `spi_prepare`. If the query references parameters, a value-list must be supplied. This is a Tcl list of actual values for the parameters. The list must be the same length as the parameter type list previously given to `spi_prepare`. Omit value-list if the query has no parameters.

  The optional value for -nulls is a string of spaces and 'n' characters telling `spi_execp` which of the parameters are null values. If given, it must have exactly the same length as the value-list. If it is not given, all the parameter values are nonnull.

  Except for the way in which the query and its parameters are specified, `spi_execp` works just like `spi_exec`. The -count, -array, and loop-body options are the same, and so is the result value.

  Here's an example of a PL/Tcl function using a prepared plan:

      CREATE FUNCTION t1_count(integer, integer) RETURNS integer AS '
          if {![ info exists GD(plan) ]} {
              # prepare the saved plan on the first call
              set GD(plan) [ spi_prepare \\
                      "SELECT count(*) AS cnt FROM t1 WHERE num >= \\$1 AND num <= \\$2" \\
                      [ list int4 int4 ] ]
          }
          spi_execp -count 1 $GD(plan) [ list $1 $2 ]
          return $cnt
      ' LANGUAGE pltcl;

  Note that each backslash that Tcl should see must be doubled when we type in the function, since the main parser processes backslashes, too, in `CREATE FUNCTION`. We need backslashes inside the query string given to `spi_prepare` to ensure that the $n markers will be passed through to `spi_prepare` as-is, and not replaced by Tcl variable substitution.

`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`. (If not, you get zero.)

`quote` string

  Duplicates all occurrences of single quote and backslash characters in the given string. This may 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`. 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. Most simply emit the given message just like the elog C function. ERROR raises an error condition: further execution of the function is abandoned, and the current transaction is aborted. 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.)

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