PL/pgSQL is a loadable procedural language for the Postgres database system. This package was originally written by Jan Wieck. The design goals of PL/pgSQL were to create a loadable procedural language that can be used to create functions and trigger procedures, adds control structures to the SQL language, can perform complex computations, inherits all user defined types, functions and operators, can be defined to be trusted by the server, is easy to use. The PL/pgSQL call handler parses the function's source text and produces an internal binary instruction tree the first time the function is called. The produced bytecode is identified in the call handler by the object ID of the function. This ensures that changing a function by a DROP/CREATE sequence will take effect without establishing a new database connection. For all expressions and SQL statements used in the function, the PL/pgSQL bytecode interpreter creates a prepared execution plan using the SPI manager's SPI_prepare() and SPI_saveplan() functions. This is done the first time the individual statement is processed in the PL/pgSQL function. Thus, a function with conditional code that contains many statements for which execution plans would be required, will only prepare and save those plans that are really used during the lifetime of the database connection. Because PL/pgSQL saves execution plans in this way, queries that appear directly in a PL/pgSQL function must refer to the same tables and fields on every execution; that is, you cannot use a parameter as the name of a table or field in a query. To get around this restriction, you can construct dynamic queries using the PL/pgSQL EXECUTE statement --- at the price of constructing a new query plan on every execution. Except for input/output conversion and calculation functions for user defined types, anything that can be defined in C language functions can also be done with PL/pgSQL. It is possible to create complex conditional computation functions and later use them to define operators or use them in functional indices. The PL/pgSQL language is case insensitive. All keywords and identifiers can be used in mixed upper- and lower-case. PL/pgSQL is a block oriented language. A block is defined as <<label>> DECLARE declarations BEGIN statements END; There can be any number of sub-blocks in the statement section of a block. Sub-blocks can be used to hide variables from outside a block of statements. The variables declared in the declarations section preceding a block are initialized to their default values every time the block is entered, not only once per function call. It is important not to confuse the use of BEGIN/END for grouping statements in PL/pgSQL with the database commands for transaction control. PL/pgSQL's BEGIN/END are only for grouping; they do not start or end a transaction. Functions and trigger procedures are always executed within a transaction established by an outer query --- they cannot start or commit transactions, since Postgres does not have nested transactions. There are two types of comments in PL/pgSQL. A double dash -- starts a comment that extends to the end of the line. A /* starts a block comment that extends to the next occurrence of */. Block comments cannot be nested, but double dash comments can be enclosed into a block comment and a double dash can hide the block comment delimiters /* and */. All variables, rows and records used in a block or its sub-blocks must be declared in the declarations section of a block, except for the loop variable of a FOR-loop iterating over a range of integer values. Parameters given to a PL/pgSQL function are automatically declared with the usual identifiers $1, $2, etc. The declarations have the following syntax: name CONSTANT type NOT NULL DEFAULT | := value ; Declares a variable of the specified base type. If the variable is declared as CONSTANT, the value cannot be changed. If NOT NULL is specified, an assignment of a NULL value results in a runtime error. Since the default value of all variables is the SQL NULL value, all variables declared as NOT NULL must also have a default value specified. The default value is evaluated every time the block is entered. So assigning 'now' to a variable of type timestamp causes the variable to have the time of the actual function call, not when the function was precompiled into its bytecode. name table%ROWTYPE; Declares a row with the structure of the given table. table must be an existing table or view name of the database. The fields of the row are accessed in the dot notation. Parameters to a function can be composite types (complete table rows). In that case, the corresponding identifier $n will be a rowtype, but it must be aliased using the ALIAS command described below. Only the user attributes of a table row are accessible in the row, no Oid or other system attributes (because the row could be from a view and view rows don't have useful system attributes). The fields of the rowtype inherit the table's field sizes or precision for char() etc. data types. name RECORD; Records are similar to rowtypes, but they have no predefined structure. They are used in selections and FOR loops to hold one actual database row from a SELECT operation. One and the same record can be used in different selections. Accessing a record or an attempt to assign a value to a record field when there is no actual row in it results in a runtime error. The NEW and OLD rows in a trigger are given to the procedure as records. This is necessary because in Postgres one and the same trigger procedure can handle trigger events for different tables. name ALIAS FOR $n; For better readability of the code it is possible to define an alias for a positional parameter to a function. This aliasing is required for composite types given as arguments to a function. The dot notation $1.salary as in SQL functions is not allowed in PL/pgSQL. RENAME oldname TO newname; Change the name of a variable, record or row. This is useful if NEW or OLD should be referenced by another name inside a trigger procedure. The type of a variable can be any of the existing base types of the database. type in the declarations section above is defined as: Postgres-basetype variable%TYPE table.field%TYPE variable is the name of a variable, previously declared in the same function, that is visible at this point. table is the name of an existing table or view where field is the name of an attribute. Using the table.field%TYPE causes PL/pgSQL to look up the attributes definitions at the first call to the function during the lifetime of a backend. Suppose we have a table with a char(20) attribute and some PL/pgSQL functions that deal with its content in local variables. Now someone decides that char(20) is not enough, dumps the table, drops it, recreates it now with the attribute in question defined as char(40) and restores the data. Hah - he forgot about the functions. The computations inside them will truncate the values to 20 characters. But if they are defined using the table.field%TYPE declarations, they will automagically handle the size change or if the new table schema defines the attribute as text type. All expressions used in PL/pgSQL statements are processed using the backend's executor. Expressions that appear to contain constants may in fact require runtime evaluation (e.g., 'now' for the timestamp type) so it is impossible for the PL/pgSQL parser to identify real constant values other than the NULL keyword. All expressions are evaluated internally by executing a query SELECT expression using the SPI manager. In the expression, occurrences of variable identifiers are substituted by parameters and the actual values from the variables are passed to the executor in the parameter array. All expressions used in a PL/pgSQL function are only prepared and saved once. The only exception to this rule is an EXECUTE statement if parsing of a query is needed each time it is encountered. The type checking done by the Postgres main parser has some side effects to the interpretation of constant values. In detail there is a difference between what the two functions CREATE FUNCTION logfunc1 (text) RETURNS timestamp AS ' DECLARE logtxt ALIAS FOR $1; BEGIN INSERT INTO logtable VALUES (logtxt, ''now''); RETURN ''now''; END; ' LANGUAGE 'plpgsql'; and CREATE FUNCTION logfunc2 (text) RETURNS timestamp AS ' DECLARE logtxt ALIAS FOR $1; curtime timestamp; BEGIN curtime := ''now''; INSERT INTO logtable VALUES (logtxt, curtime); RETURN curtime; END; ' LANGUAGE 'plpgsql'; do. In the case of logfunc1(), the Postgres main parser knows when preparing the plan for the INSERT, that the string 'now' should be interpreted as timestamp because the target field of logtable is of that type. Thus, it will make a constant from it at this time and this constant value is then used in all invocations of logfunc1() during the lifetime of the backend. Needless to say that this isn't what the programmer wanted. In the case of logfunc2(), the Postgres main parser does not know what type 'now' should become and therefore it returns a data type of text containing the string 'now'. During the assignment to the local variable curtime, the PL/pgSQL interpreter casts this string to the timestamp type by calling the text_out() and timestamp_in() functions for the conversion. This type checking done by the Postgres main parser got implemented after PL/pgSQL was nearly done. It is a difference between 6.3 and 6.4 and affects all functions using the prepared plan feature of the SPI manager. Using a local variable in the above manner is currently the only way in PL/pgSQL to get those values interpreted correctly. If record fields are used in expressions or statements, the data types of fields should not change between calls of one and the same expression. Keep this in mind when writing trigger procedures that handle events for more than one table. Anything not understood by the PL/pgSQL parser as specified below will be put into a query and sent down to the database engine to execute. The resulting query should not return any data. Assignment An assignment of a value to a variable or row/record field is written as identifier := expression; If the expressions result data type doesn't match the variables data type, or the variable has a size/precision that is known (as for char(20)), the result value will be implicitly cast by the PL/pgSQL bytecode interpreter using the result types output- and the variables type input-functions. Note that this could potentially result in runtime errors generated by the types input functions. An assignment of a complete selection into a record or row can be done by SELECT INTO target expressions FROM ...; target can be a record, a row variable or a comma separated list of variables and record-/row-fields. Note that this is quite different from Postgres' normal interpretation of SELECT INTO, which is that the INTO target is a newly created table. (If you want to create a table from a SELECT result inside a PL/pgSQL function, use the equivalent syntax CREATE TABLE AS SELECT.) if a row or a variable list is used as target, the selected values must exactly match the structure of the target(s) or a runtime error occurs. The FROM keyword can be followed by any valid qualification, grouping, sorting etc. that can be given for a SELECT statement. There is a special variable named FOUND of type boolean that can be used immediately after a SELECT INTO to check if an assignment had success. SELECT INTO myrec * FROM EMP WHERE empname = myname; IF NOT FOUND THEN RAISE EXCEPTION ''employee % not found'', myname; END IF; If the selection returns multiple rows, only the first is moved into the target fields. All others are silently discarded. Calling another function All functions defined in a Postgres database return a value. Thus, the normal way to call a function is to execute a SELECT query or doing an assignment (resulting in a PL/pgSQL internal SELECT). But there are cases where someone is not interested in the function's result. PERFORM query executes a SELECT query over the SPI manager and discards the result. Identifiers like local variables are still substituted into parameters. Executing dynamic queries EXECUTE query-string where query-string is a string of type text containing the query to be executed. Unlike all other queries in PL/pgSQL, a query run by an EXECUTE statement is not prepared and saved just once during the life of the server. Instead, the query is prepared each time the statement is run. The query-string can be dynamically created within the procedure to perform actions on variable tables and fields. The results from SELECT queries are discarded by EXECUTE, and SELECT INTO is not currently supported within EXECUTE. So, the only way to extract a result from a dynamically-created SELECT is to use the FOR ... EXECUTE form described later. An example: EXECUTE ''UPDATE tbl SET '' || quote_ident(fieldname) || '' = '' || quote_literal(newvalue) || '' WHERE ...''; This example shows use of the functions quote_ident(TEXT) and quote_literal(TEXT). Variables containing field and table identifiers should be passed to function quote_ident(). Variables containing literal elements of the dynamic query string should be passed to quote_literal(). Both take the appropriate steps to return the input text enclosed in single or double quotes and with any embedded special characters properly escaped. Obtaining other results status GET DIAGNOSTICS variable = item , ... This command allows retrieval of system status indicators. Each item is a keyword identifying a state value to be assigned to the specified variable (which should be of the right datatype to receive it). The currently available status items are ROW_COUNT, the number of rows processed by the last SQL query sent down to the SQL engine; and RESULT_OID, the Oid of the last row inserted by the most recent SQL query. Note that RESULT_OID is only useful after an INSERT query. Returning from the function RETURN expression The function terminates and the value of expression will be returned to the upper executor. The return value of a function cannot be undefined. If control reaches the end of the top-level block of the function without hitting a RETURN statement, a runtime error will occur. The expressions result will be automatically casted into the function's return type as described for assignments. Aborting and messages As indicated in the above examples there is a RAISE statement that can throw messages into the Postgres elog mechanism. RAISE level 'format' , identifier ...; Inside the format, % is used as a placeholder for the subsequent comma-separated identifiers. Possible levels are DEBUG (silently suppressed in production running databases), NOTICE (written into the database log and forwarded to the client application) and EXCEPTION (written into the database log and aborting the transaction). Conditionals IF expression THEN statements ELSE statements END IF; The expression must return a value that is of type boolean or can be casted to a boolean. Loops There are multiple types of loops. <<label>> LOOP statements END LOOP; An unconditional loop that must be terminated explicitly by an EXIT statement. The optional label can be used by EXIT statements of nested loops to specify which level of nesting should be terminated. <<label>> WHILE expression LOOP statements END LOOP; A conditional loop that is executed as long as the evaluation of expression is true. <<label>> FOR name IN REVERSE expression .. expression LOOP statements END LOOP; A loop that iterates over a range of integer values. The variable name is automatically created as type integer and exists only inside the loop. The two expressions giving the lower and upper bound of the range are evaluated only when entering the loop. The iteration step is always 1. <<label>> FOR record | row IN select_clause LOOP statements END LOOP; The record or row is assigned all the rows resulting from the select clause and the loop body is executed for each row. If the loop is terminated with an EXIT statement, the last assigned row is still accessible after the loop. <<label>> FOR record | row IN EXECUTE text_expression LOOP statements END LOOP; This is like the previous form, except that the source SELECT statement is specified as a string expression, which is evaluated and re-planned on each entry to the FOR loop. This allows the programmer to choose the speed of a pre-planned query or the flexibility of a dynamic query, just as with a plain EXECUTE statement. EXIT label WHEN expression ; If no label given, the innermost loop is terminated and the statement following END LOOP is executed next. If label is given, it must be the label of the current or an upper level of nested loop blocks. Then the named loop or block is terminated and control continues with the statement after the loops/blocks corresponding END. PL/pgSQL can be used to define trigger procedures. They are created with the usual CREATE FUNCTION command as a function with no arguments and a return type of OPAQUE. There are some Postgres specific details in functions used as trigger procedures. First they have some special variables created automatically in the top-level blocks declaration section. They are NEW Data type RECORD; variable holding the new database row on INSERT/UPDATE operations on ROW level triggers. OLD Data type RECORD; variable holding the old database row on UPDATE/DELETE operations on ROW level triggers. TG_NAME Data type name; variable that contains the name of the trigger actually fired. TG_WHEN Data type text; a string of either 'BEFORE' or 'AFTER' depending on the triggers definition. TG_LEVEL Data type text; a string of either 'ROW' or 'STATEMENT' depending on the triggers definition. TG_OP Data type text; a string of 'INSERT', 'UPDATE', or 'DELETE' telling for which operation the trigger is actually fired. TG_RELID Data type oid; the object ID of the table that caused the trigger invocation. TG_RELNAME Data type name; the name of the table that caused the trigger invocation. TG_NARGS Data type integer; the number of arguments given to the trigger procedure in the CREATE TRIGGER statement. TG_ARGV[] Data type array of text; the arguments from the CREATE TRIGGER statement. The index counts from 0 and can be given as an expression. Invalid indices (< 0 or >= tg_nargs) result in a NULL value. Second they must return either NULL or a record/row containing exactly the structure of the table the trigger was fired for. Triggers fired AFTER might always return a NULL value with no effect. Triggers fired BEFORE signal the trigger manager to skip the operation for this actual row when returning NULL. Otherwise, the returned record/row replaces the inserted/updated row in the operation. It is possible to replace single values directly in NEW and return that or to build a complete new record/row to return. Postgres does not have a very smart exception handling model. Whenever the parser, planner/optimizer or executor decide that a statement cannot be processed any longer, the whole transaction gets aborted and the system jumps back into the main loop to get the next query from the client application. It is possible to hook into the error mechanism to notice that this happens. But currently it is impossible to tell what really caused the abort (input/output conversion error, floating point error, parse error). And it is possible that the database backend is in an inconsistent state at this point so returning to the upper executor or issuing more commands might corrupt the whole database. And even if, at this point the information, that the transaction is aborted, is already sent to the client application, so resuming operation does not make any sense. Thus, the only thing PL/pgSQL currently does when it encounters an abort during execution of a function or trigger procedure is to write some additional DEBUG level log messages telling in which function and where (line number and type of statement) this happened. Here are only a few functions to demonstrate how easy it is to write PL/pgSQL functions. For more complex examples the programmer might look at the regression test for PL/pgSQL. One painful detail in writing functions in PL/pgSQL is the handling of single quotes. The function's source text in the CREATE FUNCTION command must be a literal string. Single quotes inside of literal strings must be either doubled or quoted with a backslash. We are still looking for an elegant alternative. In the meantime, doubling the single quotes as in the examples below should be used. Any solution for this in future versions of Postgres will be forward compatible. The following two PL/pgSQL functions are identical to their counterparts from the C language function discussion. CREATE FUNCTION add_one (integer) RETURNS integer AS ' BEGIN RETURN $1 + 1; END; ' LANGUAGE 'plpgsql'; CREATE FUNCTION concat_text (text, text) RETURNS text AS ' BEGIN RETURN $1 || $2; END; ' LANGUAGE 'plpgsql'; Again, this is the PL/pgSQL equivalent to the example from the C functions. CREATE FUNCTION c_overpaid (EMP, integer) RETURNS boolean AS ' DECLARE emprec ALIAS FOR $1; sallim ALIAS FOR $2; BEGIN IF emprec.salary ISNULL THEN RETURN ''f''; END IF; RETURN emprec.salary > sallim; END; ' LANGUAGE 'plpgsql'; This trigger ensures that any time a row is inserted or updated in the table, the current user name and time are stamped into the row. And it ensures that an employees name is given and that the salary is a positive value. CREATE TABLE emp ( empname text, salary integer, last_date timestamp, last_user text); CREATE FUNCTION emp_stamp () RETURNS OPAQUE AS ' BEGIN -- Check that empname and salary are given IF NEW.empname ISNULL THEN RAISE EXCEPTION ''empname cannot be NULL value''; END IF; IF NEW.salary ISNULL THEN RAISE EXCEPTION ''% cannot have NULL salary'', NEW.empname; END IF; -- Who works for us when she must pay for? IF NEW.salary < 0 THEN RAISE EXCEPTION ''% cannot have a negative salary'', NEW.empname; END IF; -- Remember who changed the payroll when NEW.last_date := ''now''; NEW.last_user := current_user; RETURN NEW; END; ' LANGUAGE 'plpgsql'; CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp FOR EACH ROW EXECUTE PROCEDURE emp_stamp();