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.
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.
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.
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_preparequerytypelist
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.
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 $n markers will be passed through to
spi_prepare as-is, and not
replaced by Tcl variable substitution.
spi_lastoid
Returns the OID of the row inserted by the last
spi_exec'd or spi_execp'd query,
if that query was a single-row INSERT. (If not, you get zero.)
quotestring
Duplicates all occurrences of single quote and backslash characters
in the given string. This may be used to safely quote strings
that are to be inserted into SQL queries given
to spi_exec or
spi_prepare.
For example, think about a query string like
"SELECT '$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.
eloglevelmsg
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).
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.
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.
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.
