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August version of docs superceded by sgml version from Jan.
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Thomas G. Lockhart
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PL/pgSQL
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A procedural language for the PostgreSQL RDBMS
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Jan Wieck <jwieck@debis.com>
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Preface
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PL/pgSQL is a procedural language based on SQL designed for
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the PostgreSQL database system.
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The extensibility features of PostgreSQL are mostly based on
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the ability to define functions for various operations.
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Functions could have been written in PostgreSQL's SQL dialect
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or in the C programming language. Functions written in C are
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compiled into a shared object and loaded by the database
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backend process on demand. Also the trigger features of
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PostgreSQL are based on functions but required the use of the
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C language.
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Since version 6.3 PostgreSQL supports the definition of
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procedural languages. In the case of a function or trigger
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procedure defined in a procedural language, the database has
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no builtin knowlege how to interpret the functions source
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text. Instead, the function and trigger calls are passed into
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a handler that knows the details of the language. The
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handler itself is a function compiled into a shared object
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and loaded on demand.
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Overview
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The PL/pgSQL language is case insensitive. All keywords and
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identifiers can be used in upper-/lowercase mixed.
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PL/pgSQL is a block oriented language. A block is defined as
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[<<label>>]
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[DECLARE
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-- declarations]
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BEGIN
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-- statements
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END;
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There can be any number of subblocks in the statements
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section of a block. Subblocks can be used to hide variables
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from outside a block of statements (see Scope and visability
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below). The variables declared in the declarations section
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preceding a block are initialized to their default values
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every time the block is entered, not only once per function
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call.
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It is important not to misunderstand the meaning of BEGIN/END
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for grouping statements in PL/pgSQL and the database commands
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for transaction control. Functions or trigger procedures
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cannot start or commit transactions and PostgreSQL
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transactions cannot have subtransactions.
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Comments
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There are two types of comments in PL/pgSQL. A double dash
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'--' starts a comment that extends to the end of the line. A
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'/*' starts a block comment that extends to the next '*/'.
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Block comments cannot be nested, but double dash comments can
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be enclosed into a block comment and double dashes can hide
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'/*' and '*/'.
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Declarations
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All variables, rows and records used in a block or it's
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subblocks must be declared in the declarations section of the
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block except for the loop variable of a FOR loop iterating
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over a range of integer values. The parameters given to the
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function are automatically declared with the usual
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identifiers $n. The declarations have the following syntax:
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<name> [CONSTANT] <type> [NOT NULL]
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[DEFAULT | := <value>];
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Declares a variable of the specified type. If the
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variable is declared as CONSTANT, the value cannot be
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changed. If NOT NULL is specified, an assignment of a
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NULL value results in a runtime error. Since the
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default value of a variable is the SQL NULL value,
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all variables declared as NOT NULL must also have a
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default value.
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The default value is evaluated at the actual function
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call. So assigning 'now' to an abstime variable
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causes the variable to have the time of the actual
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function call, not when the function was compiled
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(during it's first call since the lifetime of the
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database connection).
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<name> <class>%ROWTYPE;
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Declares a row with the structure of the given class.
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Class must be an existing table- or viewname of the
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database. The fields of the row are accessed in the
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dot notation. Parameters to a procedure could be
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tuple types. In that case the corresponding
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identifier $n will be a rowtype. Only the user
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attributes of a tuple are accessible in the row.
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There must be no whitespaces between the classname,
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the percent and the ROWTYPE keyword.
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The fields of the rowtype inherit the tables
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fieldsizes for char() etc. data types (atttypmod
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from pg_attribute).
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<name> RECORD;
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Records are similar to rowtypes, but they have no
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predefined structure They are used in selections and
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FOR loops to hold one actual database tuple from a
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select operation. One and the same record can be used
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in different selections. Accessing a record or an
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attempt to assign a value to a record field when
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there's no actual tuple in it results in a runtime
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error.
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The new and old tuples in triggers are given to the
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trigger procedure as records. This is necessary,
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because under PostgreSQL one and the same trigger
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procedure can handle trigger events for different
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tables.
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<name> ALIAS FOR $n;
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For better readability of the code it's possible to
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define an alias for a positional parameter to the
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function.
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RENAME <oldname> TO <newname>;
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Change the name of a variable, record or rowtype.
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This is useful if new or old should be referenced by
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another name inside a trigger procedure.
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Datatypes
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The type of a variable can be any of the existing data types
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of the database. <type> above is defined as:
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postgesql-basetype
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or variable%TYPE
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or rowtype.field%TYPE
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or class.field%TYPE
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As for the rowtype declaration, there must be no whitespaces
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between the classname, the percent and the TYPE keyword.
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Expressions
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All expressions used in PL/pgSQL statements are processed
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using the backends executor. Since even a constant looking
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expression can have a totally different meaning for a
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particular data type (as 'now' for abstime), it is impossible
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for the PL/pgSQL parser to identify real constant values
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other than the NULL keyword. The expressions are evaluated by
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internally executing a query
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SELECT <expr>
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over the SPI manager. In the expression, occurences of
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variable identifiers are substituted by parameters and the
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actual values from the variables are passed to the executor
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as query parameters. All the expressions used in a PL/pgSQL
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function are only prepared and saved once.
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If record fields are used in expressions or database
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statements, the data types of the fields should not change
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between calls of one and the same expression. Keep this in
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mind when writing trigger procedures that handle events for
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more than one table.
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Statements
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Anything not understood by the parser as specified below will
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be put into a query and sent down to the database engine to
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execute. The resulting query should not return any data
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(insert, update, delete queries and all utility statements).
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Assignment
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An assignment of a value to a variable or rowtype field
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is written as:
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<identifier> := <expr>;
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If the expressions result data type doesn't match the
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variables data type, or the variables atttypmod value is
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known (as for char(20)), the result value will be
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implicitly casted by the PL/pgSQL executor using the
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result types output- and the variables type input-
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functions. Note that this could potentially result in
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runtime errors generated by the types input functions.
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An assignment of a complete selection into a record or
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rowtype can be done as:
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SELECT expressions INTO <target> FROM fromlist;
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Target can be a record or rowtype variable, or a comma
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separated list of variables and record/row fields.
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If a rowtype or a variable list is used as target, the
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selected values must exactly match the structure of the
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target(s) or a runtime error occurs. The fromlist can be
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followed by any valid qualification, grouping, sorting
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etc.
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There is a special condition [NOT] FOUND that can be used
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immediately after a SELECT INTO to check if the data has
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been found.
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SELECT * INTO myrec FROM EMP WHERE empname = myname;
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IF NOT FOUND THEN
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RAISE EXCEPTION 'employee % not found', myname;
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END IF;
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If the selection returns multiple rows, only the first is
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moved into the target fields. All others are discarded.
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Calling another function
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If a function should be called, this is normally done by
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a SELECT query. But there are cases where someone isn't
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interested in the functions result.
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PERFORM querystring;
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executes a 'SELECT querystring' over the SPI manager and
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discards the result.
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Returning from the function
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RETURN <expr>;
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The function terminates and the value of <expr> will be
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returned to the upper executor. The return value of a
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function cannot be undefined. If control reaches the end
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of the toplevel block of the function without hitting a
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RETURN statement, a runtime error will occur.
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Aborting and messages
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As indicated above there is an RAISE statement that can
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throw messages into the PostgreSQL elog mechanism.
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RAISE level 'format' [, identifier [...]];
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Inside the format, % can be used as a placeholder for the
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following, comma separated identifiers. The identifiers
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must specify an existing variable or row/record field.
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Conditionals
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IF <expr> THEN
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-- statements
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[ELSE
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-- statements]
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END IF;
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The expression <expr> must return a value that at least
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can be casted into a boolean.
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Loops
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There are multiple types of loops.
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[<<label>>]
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LOOP
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-- statements
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END LOOP;
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An unconditional loop that must be terminated explicitly
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by an EXIT statement. The optional label can be used by
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EXIT statements of nested loops to specify which level of
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nesting should be terminated.
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[<<label>>]
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WHILE <expr> LOOP
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-- statements
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END LOOP;
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A conditional loop that is executed as long as the
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evaluation of <expr> returns true.
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[<<label>>]
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FOR <name> IN [REVERSE] <expr>..<expr> LOOP
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-- statements
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END LOOP.
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A loop that iterates over a range of integer values. The
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variable <name> is automatically created as type integer
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and exists only inside the loop. The two expressions
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giving the lower and upper bound of the range are
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evaluated only when entering the loop. The iteration step
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is 1.
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FOR <recname|rowname> IN <select_clause> LOOP
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-- statements
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END LOOP;
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The record or row is assigned all the rows resulting from
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the select clause and the statements executed for each.
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If the loop is terminated with an EXIT statement, the
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last accessed row is still accessible in the record or
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rowtype.
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EXIT [label] [WHEN <expr>];
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If no label given, the innermost loop is terminated and
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the statement following END LOOP is executed next. If
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label is given, it must be the label of the current or an
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upper level of nested loops or blocks. Then the named
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loop or block is terminated and control continues with
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the statement after the loops/blocks corresponding END.
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Trigger procedures
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PL/pgSQL can also be used to define trigger procedures. They
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are created using CREATE FUNCTION as a function with no
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arguments and a return type of opaque.
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There are some PostgreSQL specific details in functions used
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as trigger procedures.
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First they have some special variables created above the
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toplevel statement block. These are:
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new (record)
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The new database tuple on INSERT/UPDATE operations at
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ROW level.
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old (record)
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The old database tuple on UPDATE/DELETE operations at
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ROW level.
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tg_name (type name)
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The triggers name from pg_trigger.
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tg_when (type text)
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A string of either 'BEFORE' or 'AFTER' depending on
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the triggers definition.
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tg_level (type text)
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A string of either 'ROW' or 'STATEMENT' depending on
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the triggers definition.
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tg_op (type text)
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A string of 'INSERT', 'UPDATE' or 'DELETE' telling
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for which operation the trigger is actually fired.
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tg_relid (type oid)
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The Oid of the relation for which the trigger is
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actually fired.
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tg_relname (type name)
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The relations name for which the trigger is actually
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fired.
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tg_nargs (type integer)
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The number of arguments given to the trigger
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procedure in the CREATE TRIGGER statement.
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tg_argv[] (types text)
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The arguments from the CREATE TRIGGER statement. The
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index counts from 0 and can be given as expression.
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Invalid indices (< 0 or >= tg_nargs) result in a NULL
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value.
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Second, they must return either NULL, or a record/rowtype
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containing exactly the structure of the table the trigger was
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fired for. Triggers fired AFTER might allways return NULL
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with no effect. Triggers fired BEFORE signal the trigger
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manager to skip the operation for this actual row when
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returning NULL. Otherwise, the returned record/rowtype
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replaces the inserted/updated tuple in the operation. It is
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possible to replace single values directly in new and return
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that, or to build a complete new record/rowtype to return.
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Exceptions
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PostgreSQL doesn't have a very smart exception handling
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model. Whenever the parser, planner/optimizer or executor
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decide that a statement cannot be processed any longer, the
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whole transaction gets aborted and the the system jumps back
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into the mainloop using longjmp() to get the next query from
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the client application.
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It is possible to hook into the longjmp() mechanism to notice
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that this happens. But currently it's impossible to tell what
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really caused the abort (input/output conversion error,
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floating point error, parse error) And it's possible that the
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backend is in an inconsistent state at this point so
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returning to the upper executor or issuing more commands
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might corrupt the whole database.
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Thus, the only thing PL/pgSQL currently does when it
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encounters an abort during execution of a function or trigger
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procedure is to write some additional DEBUG log messages
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telling in which function and where (line number and type of
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statement) this happened.
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This might change in the future.
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