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 elements of the named associative array, with the
        column names used as array indexes.  In addition, the current row
        number within the result (counting from zero) is stored into the array
        element named “.tupno”, unless that name is
        in use as a column name in the result.
       
        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 or array elements; 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 or array elements 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 typelistPrepares and saves a query plan for later execution. The saved plan will be retained for the life of the current session.
        The query can 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 ... $.
        If the query uses parameters, the names of the parameter types
        must be given as a Tcl list.  (Write an empty list for
        ntypelist if no parameters are used.)
       
        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;
        We need backslashes inside the query string given to
        spi_prepare to ensure that the
        $ markers will be passed
        through to nspi_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 and the modified
        table contained OIDs.  (If not, you get zero.)
       
subtransaction command        The Tcl script contained in command is
        executed within a SQL subtransaction.  If the script returns an
        error, that entire subtransaction is rolled back before returning the
        error out to the surrounding Tcl code.
        See Section 43.9 for more details and an
        example.
       
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 19
        and Section 43.8
        for more information.