PL/Tcl Tcl PL/Tcl is a loadable procedural language for the PostgreSQL database system that enables the Tcl language to be used to write functions and trigger procedures. PL/Tcl offers most of the capabilities a function writer has in the C language, with a few restrictions, and with the addition of the powerful string processing libraries that are available for Tcl. One compelling good restriction is that everything is executed from within the safety of the context of a Tcl interpreter. In addition to the limited command set of safe Tcl, only a few commands are available to access the database via SPI and to raise messages via elog(). PL/Tcl provides no way to access internals of the database server or to gain OS-level access under the permissions of the PostgreSQL server process, as a C function can do. Thus, unprivileged database users can be trusted to use this language; it does not give them unlimited authority. The other notable implementation restriction is that Tcl functions cannot be used to create input/output functions for new data types. Sometimes it is desirable to write Tcl functions that are not restricted to safe Tcl. For example, one might want a Tcl function that sends email. To handle these cases, there is a variant of PL/Tcl called PL/TclU (for untrusted Tcl). This is exactly the same language except that a full Tcl interpreter is used. If PL/TclU is used, it must be installed as an untrusted procedural language so that only database superusers can create functions in it. The writer of a PL/TclU function must take care that the function cannot be used to do anything unwanted, since it will be able to do anything that could be done by a user logged in as the database administrator. The shared object code for the PL/Tcl and PL/TclU call handlers is automatically built and installed in the PostgreSQL library directory if Tcl support is specified in the configuration step of the installation procedure. To install PL/Tcl and/or PL/TclU in a particular database, use the CREATE EXTENSION command, for example CREATE EXTENSION pltcl or CREATE EXTENSION pltclu. 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 has to 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 to the Tcl script as variables named 1 ... n. 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 STRICT; Note the clause STRICT, which saves us from having to think about null input values: if a null value is passed, the function will not be called at all, but will just return a null result automatically. In a nonstrict 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 nonnull argument to return the nonnull 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 function 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: CREATE TABLE employee ( name text, salary integer, age integer ); CREATE FUNCTION overpaid(employee) RETURNS boolean AS $$ if {200000.0 < $1(salary)} { return "t" } if {$1(age) < 30 && 100000.0 < $1(salary)} { return "t" } return "f" $$ LANGUAGE pltcl; PL/Tcl functions can return composite-type results, too. To do this, the Tcl code must return a list of column name/value pairs matching the expected result type. Any column names omitted from the list are returned as nulls, and an error is raised if there are unexpected column names. Here is an example: CREATE FUNCTION square_cube(in int, out squared int, out cubed int) AS $$ return [list squared [expr {$1 * $1}] cubed [expr {$1 * $1 * $1}]] $$ LANGUAGE pltcl; The result list can be made from an array representation of the desired tuple with the array get Tcl command. For example: CREATE FUNCTION raise_pay(employee, delta int) RETURNS employee AS $$ set 1(salary) [expr {$1(salary) + $2}] return [array get 1] $$ LANGUAGE pltcl; PL/Tcl functions can return sets. To do this, the Tcl code should call return_next once per row to be returned, passing either the appropriate value when returning a scalar type, or a list of column name/value pairs when returning a composite type. Here is an example returning a scalar type: CREATE FUNCTION sequence(int, int) RETURNS SETOF int AS $$ for {set i $1} {$i < $2} {incr i} { return_next $i } $$ LANGUAGE pltcl; and here is one returning a composite type: CREATE FUNCTION table_of_squares(int, int) RETURNS TABLE (x int, x2 int) AS $$ for {set i $1} {$i < $2} {incr i} { return_next [list x $i x2 [expr {$i * $i}]] } $$ LANGUAGE pltcl; The argument values supplied to a PL/Tcl function's code are simply the input arguments converted to text form (just as if they had been displayed by a SELECT statement). Conversely, the return and return_next commands will accept any string that is acceptable input format for the function's declared result type, or for the specified column of a composite result type. global data in PL/Tcl Sometimes it is useful to have some global data that is held between two calls to a function or is shared between different functions. This is easily done in PL/Tcl, but there are some restrictions that must be understood. For security reasons, PL/Tcl executes functions called by any one SQL role in a separate Tcl interpreter for that role. This prevents accidental or malicious interference by one user with the behavior of another user's PL/Tcl functions. Each such interpreter will have its own values for any global Tcl variables. Thus, two PL/Tcl functions will share the same global variables if and only if they are executed by the same SQL role. In an application wherein a single session executes code under multiple SQL roles (via SECURITY DEFINER functions, use of SET ROLE, etc) you may need to take explicit steps to ensure that PL/Tcl functions can share data. To do that, make sure that functions that should communicate are owned by the same user, and mark them SECURITY DEFINER. You must of course take care that such functions can't be used to do anything unintended. All PL/TclU functions used in a session execute in the same Tcl interpreter, which of course is distinct from the interpreter(s) used for PL/Tcl functions. So global data is automatically shared between PL/TclU functions. This is not considered a security risk because all PL/TclU functions execute at the same trust level, namely that of a database superuser. To help protect PL/Tcl functions from unintentionally interfering with each other, a global array is made available to each function via the upvar command. The global name of this variable is the function's internal name, and the local name is GD. It is recommended that GD be used for persistent private data of a function. Use regular Tcl global variables only for values that you specifically intend to be shared among multiple functions. (Note that the GD arrays are only global within a particular interpreter, so they do not bypass the security restrictions mentioned above.) An example of using GD appears in the spi_execp example below. 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 typelist Prepares and saves a query plan for later execution. The saved plan will be retained for the life of the current session.preparing a query in PL/Tcl 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 ... $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.) 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 $n markers will be passed through to spi_prepare as-is, and not replaced by Tcl variable substitution. spi_lastoid spi_lastoid in PL/Tcl Returns the OID of the row inserted by the last spi_exec or spi_execp, if the command was a single-row INSERT and the modified table contained OIDs. (If not, you get zero.) subtransaction command The Tcl script contained in command is executed within a SQL subtransaction. If the script returns an error, that entire subtransaction is rolled back before returning the error out to the surrounding Tcl code. See for more details and an example. quote string Doubles all occurrences of single quote and backslash characters in the given string. This can be used to safely quote strings that are to be inserted into SQL commands given to spi_exec or spi_prepare. For example, think about an SQL command string like: "SELECT '$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 elog in PL/Tcl 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 and configuration variables. See and for more information. trigger in PL/Tcl Trigger procedures can be written in PL/Tcl. PostgreSQL requires that a procedure that is 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_table_name The name of the table that caused the trigger procedure to be invoked. $TG_table_schema The schema of the table that caused the trigger procedure to be invoked. $TG_relatts A Tcl list of the table column names, prefixed with an empty list element. So looking up a column 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 columns are customarily numbered in PostgreSQL. (Empty list elements also appear in the positions of columns that have been dropped, so that the attribute numbering is correct for columns to their right.) $TG_when The string BEFORE, AFTER, or INSTEAD OF, depending on the type of trigger event. $TG_level The string ROW or STATEMENT depending on the type of trigger event. $TG_op The string INSERT, UPDATE, DELETE, or TRUNCATE depending on the type of trigger event. $NEW An associative array containing the values of the new table row for INSERT or UPDATE actions, or empty for DELETE. The array is indexed by column name. Columns that are null will not appear in the array. This is not set for statement-level triggers. $OLD An associative array containing the values of the old table row for UPDATE or DELETE actions, or empty for INSERT. The array is indexed by column name. Columns that are null will not appear in the array. This is not set for statement-level triggers. $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 of column name/value pairs. 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; the contents of the modified row are specified by the column names and values in the list. Any columns not mentioned in the list are set to null. Returning a modified row is only meaningful for row-level BEFORE INSERT or UPDATE triggers, for which the modified row will be inserted instead of the one given in $NEW; or for row-level INSTEAD OF INSERT or UPDATE triggers where the returned row is used as the source data for INSERT RETURNING or UPDATE RETURNING clauses. In row-level BEFORE DELETE or INSTEAD OF DELETE triggers, returning a modified row has the same effect as returning OK, that is the operation proceeds. The trigger return value is ignored for all other types of triggers. The result list can be made from an array representation of the modified tuple with the array get Tcl command. 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 reused with different tables. event trigger in PL/Tcl Event trigger procedures can be written in PL/Tcl. PostgreSQL requires that a procedure that is to be called as an event trigger must be declared as a function with no arguments and a return type of event_trigger. The information from the trigger manager is passed to the procedure body in the following variables: $TG_event The name of the event the trigger is fired for. $TG_tag The command tag for which the trigger is fired. The return value of the trigger procedure is ignored. Here's a little example event trigger procedure that simply raises a NOTICE message each time a supported command is executed: CREATE OR REPLACE FUNCTION tclsnitch() RETURNS event_trigger AS $$ elog NOTICE "tclsnitch: $TG_event $TG_tag" $$ LANGUAGE pltcl; CREATE EVENT TRIGGER tcl_a_snitch ON ddl_command_start EXECUTE PROCEDURE tclsnitch(); exceptions in PL/Tcl Tcl code within or called from a PL/Tcl function can raise an error, either by executing some invalid operation or by generating an error using the Tcl error command or PL/Tcl's elog command. Such errors can be caught within Tcl using the Tcl catch command. If an error is not caught but is allowed to propagate out to the top level of execution of the PL/Tcl function, it is reported as a SQL error in the function's calling query. Conversely, SQL errors that occur within PL/Tcl's spi_exec, spi_prepare, and spi_execp commands are reported as Tcl errors, so they are catchable by Tcl's catch command. (Each of these PL/Tcl commands runs its SQL operation in a subtransaction, which is rolled back on error, so that any partially-completed operation is automatically cleaned up.) Again, if an error propagates out to the top level without being caught, it turns back into a SQL error. Tcl provides an errorCode variable that can represent additional information about an error in a form that is easy for Tcl programs to interpret. The contents are in Tcl list format, and the first word identifies the subsystem or library reporting the error; beyond that the contents are left to the individual subsystem or library. For database errors reported by PL/Tcl commands, the first word is POSTGRES, the second word is the Postgres version number, and additional words are field name/value pairs providing detailed information about the error. Fields SQLSTATE, condition, and message are always supplied (the first two represent the error code and condition name as shown in ). Fields that may be present include detail, hint, context, schema, table, column, datatype, constraint, statement, cursor_position, filename, lineno, and funcname. A convenient way to work with PL/Tcl's errorCode information is to load it into an array, so that the field names become array subscripts. Code for doing that might look like if {[catch { spi_exec $sql_command }]} { if {[lindex $::errorCode 0] == "POSTGRES"} { array set errorArray $::errorCode if {$errorArray(condition) == "undefined_table"} { # deal with missing table } else { # deal with some other type of SQL error } } } (The double colons explicitly specify that errorCode is a global variable.) subtransactions in PL/Tcl Recovering from errors caused by database access as described in can lead to an undesirable situation where some operations succeed before one of them fails, and after recovering from that error the data is left in an inconsistent state. PL/Tcl offers a solution to this problem in the form of explicit subtransactions. Consider a function that implements a transfer between two accounts: CREATE FUNCTION transfer_funds() RETURNS void AS $$ if [catch { spi_exec "UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'" spi_exec "UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'" } errormsg] { set result [format "error transferring funds: %s" $errormsg] } else { set result "funds transferred successfully" } spi_exec "INSERT INTO operations (result) VALUES ('[quote $result]')" $$ LANGUAGE pltcl; If the second UPDATE statement results in an exception being raised, this function will log the failure, but the result of the first UPDATE will nevertheless be committed. In other words, the funds will be withdrawn from Joe's account, but will not be transferred to Mary's account. This happens because each spi_exec is a separate subtransaction, and only one of those subtransactions got rolled back. To handle such cases, you can wrap multiple database operations in an explicit subtransaction, which will succeed or roll back as a whole. PL/Tcl provides a subtransaction command to manage this. We can rewrite our function as: CREATE FUNCTION transfer_funds2() RETURNS void AS $$ if [catch { subtransaction { spi_exec "UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'" spi_exec "UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'" } } errormsg] { set result [format "error transferring funds: %s" $errormsg] } else { set result "funds transferred successfully" } spi_exec "INSERT INTO operations (result) VALUES ('[quote $result]')" $$ LANGUAGE pltcl; Note that use of catch is still required for this purpose. Otherwise the error would propagate to the top level of the function, preventing the desired insertion into the operations table. The subtransaction command does not trap errors, it only assures that all database operations executed inside its scope will be rolled back together when an error is reported. A rollback of an explicit subtransaction occurs on any error reported by the contained Tcl code, not only errors originating from database access. Thus a regular Tcl exception raised inside a subtransaction command will also cause the subtransaction to be rolled back. However, non-error exits out of the contained Tcl code (for instance, due to return) do not cause a rollback. This section lists configuration parameters that affect PL/Tcl. pltcl.start_proc (string) pltcl.start_proc configuration parameter This parameter, if set to a nonempty string, specifies the name (possibly schema-qualified) of a parameterless PL/Tcl function that is to be executed whenever a new Tcl interpreter is created for PL/Tcl. Such a function can perform per-session initialization, such as loading additional Tcl code. A new Tcl interpreter is created when a PL/Tcl function is first executed in a database session, or when an additional interpreter has to be created because a PL/Tcl function is called by a new SQL role. The referenced function must be written in the pltcl language, and must not be marked SECURITY DEFINER. (These restrictions ensure that it runs in the interpreter it's supposed to initialize.) The current user must have permission to call it, too. If the function fails with an error it will abort the function call that caused the new interpreter to be created and propagate out to the calling query, causing the current transaction or subtransaction to be aborted. Any actions already done within Tcl won't be undone; however, that interpreter won't be used again. If the language is used again the initialization will be attempted again within a fresh Tcl interpreter. Only superusers can change this setting. Although this setting can be changed within a session, such changes will not affect Tcl interpreters that have already been created. pltclu.start_proc (string) pltclu.start_proc configuration parameter This parameter is exactly like pltcl.start_proc, except that it applies to PL/TclU. The referenced function must be written in the pltclu language. In PostgreSQL, the same function name can be used for different function definitions 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 function from the system table pg_proc 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.