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20.2. Description

20.2.1. PL/Tcl Functions and Arguments

To create a function in the PL/Tcl language, use the standard syntax

CREATE FUNCTION funcname (argument-types) RETURNS return-type AS '
    # PL/Tcl function body
' LANGUAGE 'pltcl';
     

PL/TclU is the same, except that the language should be specified as pltclu.

The body of the function is simply a piece of Tcl script. When the function is called, the argument values are passed as variables $1 ... $n to the Tcl script. The result is returned from the Tcl code in the usual way, with a return statement. For example, a function returning the greater of two integer values could be defined as:

CREATE FUNCTION tcl_max (integer, integer) RETURNS integer AS '
    if {$1 > $2} {return $1}
    return $2
' LANGUAGE 'pltcl' WITH (isStrict);
     

Note the clause WITH (isStrict), which saves us from having to think about NULL input values: if a NULL is passed, the function will not be called at all, but will just return a NULL result automatically.

In a non-strict function, if the actual value of an argument is NULL, the corresponding $n variable will be set to an empty string. To detect whether a particular argument is NULL, use the function argisnull. For example, suppose that we wanted tcl_max with one null and one non-null argument to return the non-null argument, rather than NULL:

CREATE FUNCTION tcl_max (integer, integer) RETURNS integer AS '
    if {[argisnull 1]} {
        if {[argisnull 2]} { return_null }
        return $2
    }
    if {[argisnull 2]} { return $1 }
    if {$1 > $2} {return $1}
    return $2
' LANGUAGE 'pltcl';
     

As shown above, to return a NULL value from a PL/Tcl function, execute return_null. This can be done whether the function is strict or not.

Composite-type arguments are passed to the procedure as Tcl arrays. The element names of the array are the attribute names of the composite type. If an attribute in the passed row has the NULL value, it will not appear in the array! Here is an example that defines the overpaid_2 function (as found in the older PostgreSQL documentation) in PL/Tcl:

CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
    if {200000.0 < $1(salary)} {
        return "t"
    }
    if {$1(age) < 30 && 100000.0 < $1(salary)} {
        return "t"
    }
    return "f"
' LANGUAGE 'pltcl';
     

There is not currently any support for returning a composite-type result value.

20.2.2. Data Values in PL/Tcl

The argument values supplied to a PL/Tcl function's script are simply the input arguments converted to text form (just as if they had been displayed by a SELECT statement). Conversely, the return command will accept any string that is acceptable input format for the function's declared return type. So, the PL/Tcl programmer can manipulate data values as if they were just text.

20.2.3. Global Data in PL/Tcl

Sometimes it is useful to have some global status data that is held between two calls to a procedure or is shared between different procedures. This is easily done since all PL/Tcl procedures executed in one backend share the same safe Tcl interpreter. So, any global Tcl variable is accessible to all PL/Tcl procedure calls, and will persist for the duration of the SQL client connection. (Note that PL/TclU functions likewise share global data, but they are in a different Tcl interpreter and cannot communicate with PL/Tcl functions.)

To help protect PL/Tcl procedures from unintentionally interfering with each other, a global array is made available to each procedure via the upvar command. The global name of this variable is the procedure's internal name and the local name is GD. It is recommended that GD be used for private status data of a procedure. Use regular Tcl global variables only for values that you specifically intend to be shared among multiple procedures.

An example of using GD appears in the spi_execp example below.

20.2.4. Database Access from PL/Tcl

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

spi_exec ?-count n? ?-array name? query ?loop-body?

Execute an SQL query given as a string. An error in the query causes an error to be raised. Otherwise, the command's return value is the number of rows processed (selected, inserted, updated, or deleted) by the query, or zero if the query is a utility statement. In addition, if the query 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 query. The effect of this is comparable to setting up the query as a cursor and then saying FETCH n.

If the query is a SELECT statement, the values of the statement's 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 SELECT column names used as array indexes.

If the query 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 store occurs if the SELECT 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 SELECT result (note: loop-body is ignored if the given query is not a SELECT). The values of the current row's fields 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 DEBUG 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 field of a SELECT 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 backend.

The query may use arguments, which are placeholders for values to be supplied whenever the plan is actually executed. In the query string, refer to arguments by the symbols $1 ... $n. If the query uses arguments, the names of the argument types must be given as a Tcl list. (Write an empty list for typelist if no arguments are used.) Presently, the argument types must be identified by the internal type names shown in 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?

Execute a query previously prepared with spi_prepare. queryid is the ID returned by spi_prepare. If the query references arguments, a value-list must be supplied: this is a Tcl list of actual values for the arguments. This must be the same length as the argument type list previously given to spi_prepare. Omit value-list if the query has no arguments.

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

Except for the way in which the query and its arguments 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'd or spi_execp'd query, if that query 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 queries given to spi_exec or spi_prepare. For example, think about a query string like

"SELECT '$val' AS ret"

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

SELECT 'doesn't' AS ret

which would cause a parse error during spi_exec or spi_prepare. The submitted query should contain

SELECT 'doesn''t' AS ret

which can be formed in PL/Tcl as

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

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

elog level msg

Emit 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 backend 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 backend to shut down (there is probably no good reason to use this error level in PL/Tcl functions, but it's provided for completeness).

20.2.5. Trigger Procedures in PL/Tcl

Trigger procedures can be written in PL/Tcl. As is customary in PostgreSQL, a procedure that's to be called as a trigger must be declared as a function with no arguments and a return type of trigger.

The information from the trigger manager is passed to the procedure body in the following variables:

$TG_name

The name of the trigger from the CREATE TRIGGER statement.

$TG_relid

The object ID of the table that caused the trigger procedure to be invoked.

$TG_relatts

A Tcl list of the table field names, prefixed with an empty list element. So looking up an element name in the list with Tcl's lsearch command returns the element's number starting with 1 for the first column, the same way the fields are customarily numbered in PostgreSQL.

$TG_when

The string BEFORE or AFTER depending on the type of trigger call.

$TG_level

The string ROW or STATEMENT depending on the type of trigger call.

$TG_op

The string INSERT, UPDATE or DELETE depending on the type of trigger call.

$NEW

An associative array containing the values of the new table row for INSERT/UPDATE actions, or empty for DELETE. The array is indexed by field name. Fields that are NULL will not appear in the array!

$OLD

An associative array containing the values of the old table row for UPDATE/DELETE actions, or empty for INSERT. The array is indexed by field name. Fields that are NULL will not appear in the array!

$args

A Tcl list of the arguments to the procedure as given in the CREATE TRIGGER statement. These arguments are also accessible as $1 ... $n in the procedure body.

The return value from a trigger procedure can be one of the strings OK or SKIP, or a list as returned by the array get Tcl command. If the return value is OK, the operation (INSERT/UPDATE/DELETE) that fired the trigger will proceed normally. SKIP tells the trigger manager to silently suppress the operation for this row. If a list is returned, it tells PL/Tcl to return a modified row to the trigger manager that will be inserted instead of the one given in $NEW (this works for INSERT/UPDATE only). Needless to say that all this is only meaningful when the trigger is BEFORE and FOR EACH ROW; otherwise the return value is ignored.

Here's a little example trigger procedure that forces an integer value in a table to keep track of the number of updates that are performed on the row. For new rows inserted, the value is initialized to 0 and then incremented on every update operation:

CREATE FUNCTION trigfunc_modcount() RETURNS TRIGGER AS '
    switch $TG_op {
        INSERT {
            set NEW($1) 0
        }
        UPDATE {
            set NEW($1) $OLD($1)
            incr NEW($1)
        }
        default {
            return OK
        }
    }
    return [array get NEW]
' LANGUAGE 'pltcl';

CREATE TABLE mytab (num integer, description text, modcnt integer);

CREATE TRIGGER trig_mytab_modcount BEFORE INSERT OR UPDATE ON mytab
    FOR EACH ROW EXECUTE PROCEDURE trigfunc_modcount('modcnt');
     

Notice that the trigger procedure itself does not know the column name; that's supplied from the trigger arguments. This lets the trigger procedure be re-used with different tables.

20.2.6. Modules and the unknown command

PL/Tcl has support for auto-loading Tcl code when used. It recognizes a special table, pltcl_modules, which is presumed to contain modules of Tcl code. If this table exists, the module unknown is fetched from the table and loaded into the Tcl interpreter immediately after creating the interpreter.

While the unknown module could actually contain any initialization script you need, it normally defines a Tcl "unknown" procedure that is invoked whenever Tcl does not recognize an invoked procedure name. PL/Tcl's standard version of this procedure tries to find a module in pltcl_modules that will define the required procedure. If one is found, it is loaded into the interpreter, and then execution is allowed to proceed with the originally attempted procedure call. A secondary table pltcl_modfuncs provides an index of which functions are defined by which modules, so that the lookup is reasonably quick.

The PostgreSQL distribution includes support scripts to maintain these tables: pltcl_loadmod, pltcl_listmod, pltcl_delmod, as well as source for the standard unknown module share/unknown.pltcl. This module must be loaded into each database initially to support the autoloading mechanism.

The tables pltcl_modules and pltcl_modfuncs must be readable by all, but it is wise to make them owned and writable only by the database administrator.

20.2.7. Tcl Procedure Names

In PostgreSQL, one and the same function name can be used for different functions as long as the number of arguments or their types differ. Tcl, however, requires all procedure names to be distinct. PL/Tcl deals with this by making the internal Tcl procedure names contain the object ID of the procedure's pg_proc row as part of their name. Thus, PostgreSQL functions with the same name and different argument types will be different Tcl procedures too. This is not normally a concern for a PL/Tcl programmer, but it might be visible when debugging.

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