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41.6. Control Structures #

   41.6.1. Returning from a Function
   41.6.2. Returning from a Procedure
   41.6.3. Calling a Procedure
   41.6.4. Conditionals
   41.6.5. Simple Loops
   41.6.6. Looping through Query Results
   41.6.7. Looping through Arrays
   41.6.8. Trapping Errors
   41.6.9. Obtaining Execution Location Information

   Control structures are probably the most useful (and important) part of
   PL/pgSQL. With PL/pgSQL's control structures, you can manipulate
   PostgreSQL data in a very flexible and powerful way.

41.6.1. Returning from a Function #

   There are two commands available that allow you to return data from a
   function: RETURN and RETURN NEXT.

41.6.1.1. RETURN #

RETURN expression;

   RETURN with an expression terminates the function and returns the value
   of expression to the caller. This form is used for PL/pgSQL functions
   that do not return a set.

   In a function that returns a scalar type, the expression's result will
   automatically be cast into the function's return type as described for
   assignments. But to return a composite (row) value, you must write an
   expression delivering exactly the requested column set. This may
   require use of explicit casting.

   If you declared the function with output parameters, write just RETURN
   with no expression. The current values of the output parameter
   variables will be returned.

   If you declared the function to return void, a RETURN statement can be
   used to exit the function early; but do not write an expression
   following RETURN.

   The return value of a function cannot be left undefined. If control
   reaches the end of the top-level block of the function without hitting
   a RETURN statement, a run-time error will occur. This restriction does
   not apply to functions with output parameters and functions returning
   void, however. In those cases a RETURN statement is automatically
   executed if the top-level block finishes.

   Some examples:
-- functions returning a scalar type
RETURN 1 + 2;
RETURN scalar_var;

-- functions returning a composite type
RETURN composite_type_var;
RETURN (1, 2, 'three'::text);  -- must cast columns to correct types

41.6.1.2. RETURN NEXT and RETURN QUERY #

RETURN NEXT expression;
RETURN QUERY query;
RETURN QUERY EXECUTE command-string [ USING expression [, ... ] ];

   When a PL/pgSQL function is declared to return SETOF sometype, the
   procedure to follow is slightly different. In that case, the individual
   items to return are specified by a sequence of RETURN NEXT or RETURN
   QUERY commands, and then a final RETURN command with no argument is
   used to indicate that the function has finished executing. RETURN NEXT
   can be used with both scalar and composite data types; with a composite
   result type, an entire “table” of results will be returned. RETURN
   QUERY appends the results of executing a query to the function's result
   set. RETURN NEXT and RETURN QUERY can be freely intermixed in a single
   set-returning function, in which case their results will be
   concatenated.

   RETURN NEXT and RETURN QUERY do not actually return from the function —
   they simply append zero or more rows to the function's result set.
   Execution then continues with the next statement in the PL/pgSQL
   function. As successive RETURN NEXT or RETURN QUERY commands are
   executed, the result set is built up. A final RETURN, which should have
   no argument, causes control to exit the function (or you can just let
   control reach the end of the function).

   RETURN QUERY has a variant RETURN QUERY EXECUTE, which specifies the
   query to be executed dynamically. Parameter expressions can be inserted
   into the computed query string via USING, in just the same way as in
   the EXECUTE command.

   If you declared the function with output parameters, write just RETURN
   NEXT with no expression. On each execution, the current values of the
   output parameter variable(s) will be saved for eventual return as a row
   of the result. Note that you must declare the function as returning
   SETOF record when there are multiple output parameters, or SETOF
   sometype when there is just one output parameter of type sometype, in
   order to create a set-returning function with output parameters.

   Here is an example of a function using RETURN NEXT:
CREATE TABLE foo (fooid INT, foosubid INT, fooname TEXT);
INSERT INTO foo VALUES (1, 2, 'three');
INSERT INTO foo VALUES (4, 5, 'six');

CREATE OR REPLACE FUNCTION get_all_foo() RETURNS SETOF foo AS
$BODY$
DECLARE
    r foo%rowtype;
BEGIN
    FOR r IN
        SELECT * FROM foo WHERE fooid > 0
    LOOP
        -- can do some processing here
        RETURN NEXT r; -- return current row of SELECT
    END LOOP;
    RETURN;
END;
$BODY$
LANGUAGE plpgsql;

SELECT * FROM get_all_foo();

   Here is an example of a function using RETURN QUERY:
CREATE FUNCTION get_available_flightid(date) RETURNS SETOF integer AS
$BODY$
BEGIN
    RETURN QUERY SELECT flightid
                   FROM flight
                  WHERE flightdate >= $1
                    AND flightdate < ($1 + 1);

    -- Since execution is not finished, we can check whether rows were returned
    -- and raise exception if not.
    IF NOT FOUND THEN
        RAISE EXCEPTION 'No flight at %.', $1;
    END IF;

    RETURN;
 END;
$BODY$
LANGUAGE plpgsql;

-- Returns available flights or raises exception if there are no
-- available flights.
SELECT * FROM get_available_flightid(CURRENT_DATE);

Note

   The current implementation of RETURN NEXT and RETURN QUERY stores the
   entire result set before returning from the function, as discussed
   above. That means that if a PL/pgSQL function produces a very large
   result set, performance might be poor: data will be written to disk to
   avoid memory exhaustion, but the function itself will not return until
   the entire result set has been generated. A future version of PL/pgSQL
   might allow users to define set-returning functions that do not have
   this limitation. Currently, the point at which data begins being
   written to disk is controlled by the work_mem configuration variable.
   Administrators who have sufficient memory to store larger result sets
   in memory should consider increasing this parameter.

41.6.2. Returning from a Procedure #

   A procedure does not have a return value. A procedure can therefore end
   without a RETURN statement. If you wish to use a RETURN statement to
   exit the code early, write just RETURN with no expression.

   If the procedure has output parameters, the final values of the output
   parameter variables will be returned to the caller.

41.6.3. Calling a Procedure #

   A PL/pgSQL function, procedure, or DO block can call a procedure using
   CALL. Output parameters are handled differently from the way that CALL
   works in plain SQL. Each OUT or INOUT parameter of the procedure must
   correspond to a variable in the CALL statement, and whatever the
   procedure returns is assigned back to that variable after it returns.
   For example:
CREATE PROCEDURE triple(INOUT x int)
LANGUAGE plpgsql
AS $$
BEGIN
    x := x * 3;
END;
$$;

DO $$
DECLARE myvar int := 5;
BEGIN
  CALL triple(myvar);
  RAISE NOTICE 'myvar = %', myvar;  -- prints 15
END;
$$;

   The variable corresponding to an output parameter can be a simple
   variable or a field of a composite-type variable. Currently, it cannot
   be an element of an array.

41.6.4. Conditionals #

   IF and CASE statements let you execute alternative commands based on
   certain conditions. PL/pgSQL has three forms of IF:
     * IF ... THEN ... END IF
     * IF ... THEN ... ELSE ... END IF
     * IF ... THEN ... ELSIF ... THEN ... ELSE ... END IF

   and two forms of CASE:
     * CASE ... WHEN ... THEN ... ELSE ... END CASE
     * CASE WHEN ... THEN ... ELSE ... END CASE

41.6.4.1. IF-THEN #

IF boolean-expression THEN
    statements
END IF;

   IF-THEN statements are the simplest form of IF. The statements between
   THEN and END IF will be executed if the condition is true. Otherwise,
   they are skipped.

   Example:
IF v_user_id <> 0 THEN
    UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;

41.6.4.2. IF-THEN-ELSE #

IF boolean-expression THEN
    statements
ELSE
    statements
END IF;

   IF-THEN-ELSE statements add to IF-THEN by letting you specify an
   alternative set of statements that should be executed if the condition
   is not true. (Note this includes the case where the condition evaluates
   to NULL.)

   Examples:
IF parentid IS NULL OR parentid = ''
THEN
    RETURN fullname;
ELSE
    RETURN hp_true_filename(parentid) || '/' || fullname;
END IF;

IF v_count > 0 THEN
    INSERT INTO users_count (count) VALUES (v_count);
    RETURN 't';
ELSE
    RETURN 'f';
END IF;

41.6.4.3. IF-THEN-ELSIF #

IF boolean-expression THEN
    statements
[ ELSIF boolean-expression THEN
    statements
[ ELSIF boolean-expression THEN
    statements
    ...
]
]
[ ELSE
    statements ]
END IF;

   Sometimes there are more than just two alternatives. IF-THEN-ELSIF
   provides a convenient method of checking several alternatives in turn.
   The IF conditions are tested successively until the first one that is
   true is found. Then the associated statement(s) are executed, after
   which control passes to the next statement after END IF. (Any
   subsequent IF conditions are not tested.) If none of the IF conditions
   is true, then the ELSE block (if any) is executed.

   Here is an example:
IF number = 0 THEN
    result := 'zero';
ELSIF number > 0 THEN
    result := 'positive';
ELSIF number < 0 THEN
    result := 'negative';
ELSE
    -- hmm, the only other possibility is that number is null
    result := 'NULL';
END IF;

   The key word ELSIF can also be spelled ELSEIF.

   An alternative way of accomplishing the same task is to nest
   IF-THEN-ELSE statements, as in the following example:
IF demo_row.sex = 'm' THEN
    pretty_sex := 'man';
ELSE
    IF demo_row.sex = 'f' THEN
        pretty_sex := 'woman';
    END IF;
END IF;

   However, this method requires writing a matching END IF for each IF, so
   it is much more cumbersome than using ELSIF when there are many
   alternatives.

41.6.4.4. Simple CASE #

CASE search-expression
    WHEN expression [, expression [ ... ]] THEN
      statements
  [ WHEN expression [, expression [ ... ]] THEN
      statements
    ... ]
  [ ELSE
      statements ]
END CASE;

   The simple form of CASE provides conditional execution based on
   equality of operands. The search-expression is evaluated (once) and
   successively compared to each expression in the WHEN clauses. If a
   match is found, then the corresponding statements are executed, and
   then control passes to the next statement after END CASE. (Subsequent
   WHEN expressions are not evaluated.) If no match is found, the ELSE
   statements are executed; but if ELSE is not present, then a
   CASE_NOT_FOUND exception is raised.

   Here is a simple example:
CASE x
    WHEN 1, 2 THEN
        msg := 'one or two';
    ELSE
        msg := 'other value than one or two';
END CASE;

41.6.4.5. Searched CASE #

CASE
    WHEN boolean-expression THEN
      statements
  [ WHEN boolean-expression THEN
      statements
    ... ]
  [ ELSE
      statements ]
END CASE;

   The searched form of CASE provides conditional execution based on truth
   of Boolean expressions. Each WHEN clause's boolean-expression is
   evaluated in turn, until one is found that yields true. Then the
   corresponding statements are executed, and then control passes to the
   next statement after END CASE. (Subsequent WHEN expressions are not
   evaluated.) If no true result is found, the ELSE statements are
   executed; but if ELSE is not present, then a CASE_NOT_FOUND exception
   is raised.

   Here is an example:
CASE
    WHEN x BETWEEN 0 AND 10 THEN
        msg := 'value is between zero and ten';
    WHEN x BETWEEN 11 AND 20 THEN
        msg := 'value is between eleven and twenty';
END CASE;

   This form of CASE is entirely equivalent to IF-THEN-ELSIF, except for
   the rule that reaching an omitted ELSE clause results in an error
   rather than doing nothing.

41.6.5. Simple Loops #

   With the LOOP, EXIT, CONTINUE, WHILE, FOR, and FOREACH statements, you
   can arrange for your PL/pgSQL function to repeat a series of commands.

41.6.5.1. LOOP #

[ <<label>> ]
LOOP
    statements
END LOOP [ label ];

   LOOP defines an unconditional loop that is repeated indefinitely until
   terminated by an EXIT or RETURN statement. The optional label can be
   used by EXIT and CONTINUE statements within nested loops to specify
   which loop those statements refer to.

41.6.5.2. EXIT #

EXIT [ label ] [ WHEN boolean-expression ];

   If no label is 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 some outer level of nested loop or
   block. Then the named loop or block is terminated and control continues
   with the statement after the loop's/block's corresponding END.

   If WHEN is specified, the loop exit occurs only if boolean-expression
   is true. Otherwise, control passes to the statement after EXIT.

   EXIT can be used with all types of loops; it is not limited to use with
   unconditional loops.

   When used with a BEGIN block, EXIT passes control to the next statement
   after the end of the block. Note that a label must be used for this
   purpose; an unlabeled EXIT is never considered to match a BEGIN block.
   (This is a change from pre-8.4 releases of PostgreSQL, which would
   allow an unlabeled EXIT to match a BEGIN block.)

   Examples:
LOOP
    -- some computations
    IF count > 0 THEN
        EXIT;  -- exit loop
    END IF;
END LOOP;

LOOP
    -- some computations
    EXIT WHEN count > 0;  -- same result as previous example
END LOOP;

<<ablock>>
BEGIN
    -- some computations
    IF stocks > 100000 THEN
        EXIT ablock;  -- causes exit from the BEGIN block
    END IF;
    -- computations here will be skipped when stocks > 100000
END;

41.6.5.3. CONTINUE #

CONTINUE [ label ] [ WHEN boolean-expression ];

   If no label is given, the next iteration of the innermost loop is
   begun. That is, all statements remaining in the loop body are skipped,
   and control returns to the loop control expression (if any) to
   determine whether another loop iteration is needed. If label is
   present, it specifies the label of the loop whose execution will be
   continued.

   If WHEN is specified, the next iteration of the loop is begun only if
   boolean-expression is true. Otherwise, control passes to the statement
   after CONTINUE.

   CONTINUE can be used with all types of loops; it is not limited to use
   with unconditional loops.

   Examples:
LOOP
    -- some computations
    EXIT WHEN count > 100;
    CONTINUE WHEN count < 50;
    -- some computations for count IN [50 .. 100]
END LOOP;

41.6.5.4. WHILE #

[ <<label>> ]
WHILE boolean-expression LOOP
    statements
END LOOP [ label ];

   The WHILE statement repeats a sequence of statements so long as the
   boolean-expression evaluates to true. The expression is checked just
   before each entry to the loop body.

   For example:
WHILE amount_owed > 0 AND gift_certificate_balance > 0 LOOP
    -- some computations here
END LOOP;

WHILE NOT done LOOP
    -- some computations here
END LOOP;

41.6.5.5. FOR (Integer Variant) #

[ <<label>> ]
FOR name IN [ REVERSE ] expression .. expression [ BY expression ] LOOP
    statements
END LOOP [ label ];

   This form of FOR creates a loop that iterates over a range of integer
   values. The variable name is automatically defined as type integer and
   exists only inside the loop (any existing definition of the variable
   name is ignored within the loop). The two expressions giving the lower
   and upper bound of the range are evaluated once when entering the loop.
   If the BY clause isn't specified the iteration step is 1, otherwise
   it's the value specified in the BY clause, which again is evaluated
   once on loop entry. If REVERSE is specified then the step value is
   subtracted, rather than added, after each iteration.

   Some examples of integer FOR loops:
FOR i IN 1..10 LOOP
    -- i will take on the values 1,2,3,4,5,6,7,8,9,10 within the loop
END LOOP;

FOR i IN REVERSE 10..1 LOOP
    -- i will take on the values 10,9,8,7,6,5,4,3,2,1 within the loop
END LOOP;

FOR i IN REVERSE 10..1 BY 2 LOOP
    -- i will take on the values 10,8,6,4,2 within the loop
END LOOP;

   If the lower bound is greater than the upper bound (or less than, in
   the REVERSE case), the loop body is not executed at all. No error is
   raised.

   If a label is attached to the FOR loop then the integer loop variable
   can be referenced with a qualified name, using that label.

41.6.6. Looping through Query Results #

   Using a different type of FOR loop, you can iterate through the results
   of a query and manipulate that data accordingly. The syntax is:
[ <<label>> ]
FOR target IN query LOOP
    statements
END LOOP [ label ];

   The target is a record variable, row variable, or comma-separated list
   of scalar variables. The target is successively assigned each row
   resulting from the query and the loop body is executed for each row.
   Here is an example:
CREATE FUNCTION refresh_mviews() RETURNS integer AS $$
DECLARE
    mviews RECORD;
BEGIN
    RAISE NOTICE 'Refreshing all materialized views...';

    FOR mviews IN
       SELECT n.nspname AS mv_schema,
              c.relname AS mv_name,
              pg_catalog.pg_get_userbyid(c.relowner) AS owner
         FROM pg_catalog.pg_class c
    LEFT JOIN pg_catalog.pg_namespace n ON (n.oid = c.relnamespace)
        WHERE c.relkind = 'm'
     ORDER BY 1
    LOOP

        -- Now "mviews" has one record with information about the materialized v
iew

        RAISE NOTICE 'Refreshing materialized view %.% (owner: %)...',
                     quote_ident(mviews.mv_schema),
                     quote_ident(mviews.mv_name),
                     quote_ident(mviews.owner);
        EXECUTE format('REFRESH MATERIALIZED VIEW %I.%I', mviews.mv_schema, mvie
ws.mv_name);
    END LOOP;

    RAISE NOTICE 'Done refreshing materialized views.';
    RETURN 1;
END;
$$ LANGUAGE plpgsql;

   If the loop is terminated by an EXIT statement, the last assigned row
   value is still accessible after the loop.

   The query used in this type of FOR statement can be any SQL command
   that returns rows to the caller: SELECT is the most common case, but
   you can also use INSERT, UPDATE, DELETE, or MERGE with a RETURNING
   clause. Some utility commands such as EXPLAIN will work too.

   PL/pgSQL variables are replaced by query parameters, and the query plan
   is cached for possible re-use, as discussed in detail in
   Section 41.11.1 and Section 41.11.2.

   The FOR-IN-EXECUTE statement is another way to iterate over rows:
[ <<label>> ]
FOR target IN EXECUTE text_expression [ USING expression [, ... ] ] LOOP
    statements
END LOOP [ label ];

   This is like the previous form, except that the source query is
   specified as a string expression, which is evaluated and replanned on
   each entry to the FOR loop. This allows the programmer to choose the
   speed of a preplanned query or the flexibility of a dynamic query, just
   as with a plain EXECUTE statement. As with EXECUTE, parameter values
   can be inserted into the dynamic command via USING.

   Another way to specify the query whose results should be iterated
   through is to declare it as a cursor. This is described in
   Section 41.7.4.

41.6.7. Looping through Arrays #

   The FOREACH loop is much like a FOR loop, but instead of iterating
   through the rows returned by an SQL query, it iterates through the
   elements of an array value. (In general, FOREACH is meant for looping
   through components of a composite-valued expression; variants for
   looping through composites besides arrays may be added in future.) The
   FOREACH statement to loop over an array is:
[ <<label>> ]
FOREACH target [ SLICE number ] IN ARRAY expression LOOP
    statements
END LOOP [ label ];

   Without SLICE, or if SLICE 0 is specified, the loop iterates through
   individual elements of the array produced by evaluating the expression.
   The target variable is assigned each element value in sequence, and the
   loop body is executed for each element. Here is an example of looping
   through the elements of an integer array:
CREATE FUNCTION sum(int[]) RETURNS int8 AS $$
DECLARE
  s int8 := 0;
  x int;
BEGIN
  FOREACH x IN ARRAY $1
  LOOP
    s := s + x;
  END LOOP;
  RETURN s;
END;
$$ LANGUAGE plpgsql;

   The elements are visited in storage order, regardless of the number of
   array dimensions. Although the target is usually just a single
   variable, it can be a list of variables when looping through an array
   of composite values (records). In that case, for each array element,
   the variables are assigned from successive columns of the composite
   value.

   With a positive SLICE value, FOREACH iterates through slices of the
   array rather than single elements. The SLICE value must be an integer
   constant not larger than the number of dimensions of the array. The
   target variable must be an array, and it receives successive slices of
   the array value, where each slice is of the number of dimensions
   specified by SLICE. Here is an example of iterating through
   one-dimensional slices:
CREATE FUNCTION scan_rows(int[]) RETURNS void AS $$
DECLARE
  x int[];
BEGIN
  FOREACH x SLICE 1 IN ARRAY $1
  LOOP
    RAISE NOTICE 'row = %', x;
  END LOOP;
END;
$$ LANGUAGE plpgsql;

SELECT scan_rows(ARRAY[[1,2,3],[4,5,6],[7,8,9],[10,11,12]]);

NOTICE:  row = {1,2,3}
NOTICE:  row = {4,5,6}
NOTICE:  row = {7,8,9}
NOTICE:  row = {10,11,12}

41.6.8. Trapping Errors #

   By default, any error occurring in a PL/pgSQL function aborts execution
   of the function and the surrounding transaction. You can trap errors
   and recover from them by using a BEGIN block with an EXCEPTION clause.
   The syntax is an extension of the normal syntax for a BEGIN block:
[ <<label>> ]
[ DECLARE
    declarations ]
BEGIN
    statements
EXCEPTION
    WHEN condition [ OR condition ... ] THEN
        handler_statements
    [ WHEN condition [ OR condition ... ] THEN
          handler_statements
      ... ]
END;

   If no error occurs, this form of block simply executes all the
   statements, and then control passes to the next statement after END.
   But if an error occurs within the statements, further processing of the
   statements is abandoned, and control passes to the EXCEPTION list. The
   list is searched for the first condition matching the error that
   occurred. If a match is found, the corresponding handler_statements are
   executed, and then control passes to the next statement after END. If
   no match is found, the error propagates out as though the EXCEPTION
   clause were not there at all: the error can be caught by an enclosing
   block with EXCEPTION, or if there is none it aborts processing of the
   function.

   The condition names can be any of those shown in Appendix A. A category
   name matches any error within its category. The special condition name
   OTHERS matches every error type except QUERY_CANCELED and
   ASSERT_FAILURE. (It is possible, but often unwise, to trap those two
   error types by name.) Condition names are not case-sensitive. Also, an
   error condition can be specified by SQLSTATE code; for example these
   are equivalent:
WHEN division_by_zero THEN ...
WHEN SQLSTATE '22012' THEN ...

   If a new error occurs within the selected handler_statements, it cannot
   be caught by this EXCEPTION clause, but is propagated out. A
   surrounding EXCEPTION clause could catch it.

   When an error is caught by an EXCEPTION clause, the local variables of
   the PL/pgSQL function remain as they were when the error occurred, but
   all changes to persistent database state within the block are rolled
   back. As an example, consider this fragment:
INSERT INTO mytab(firstname, lastname) VALUES('Tom', 'Jones');
BEGIN
    UPDATE mytab SET firstname = 'Joe' WHERE lastname = 'Jones';
    x := x + 1;
    y := x / 0;
EXCEPTION
    WHEN division_by_zero THEN
        RAISE NOTICE 'caught division_by_zero';
        RETURN x;
END;

   When control reaches the assignment to y, it will fail with a
   division_by_zero error. This will be caught by the EXCEPTION clause.
   The value returned in the RETURN statement will be the incremented
   value of x, but the effects of the UPDATE command will have been rolled
   back. The INSERT command preceding the block is not rolled back,
   however, so the end result is that the database contains Tom Jones not
   Joe Jones.

Tip

   A block containing an EXCEPTION clause is significantly more expensive
   to enter and exit than a block without one. Therefore, don't use
   EXCEPTION without need.

   Example 41.2. Exceptions with UPDATE/INSERT

   This example uses exception handling to perform either UPDATE or
   INSERT, as appropriate. It is recommended that applications use INSERT
   with ON CONFLICT DO UPDATE rather than actually using this pattern.
   This example serves primarily to illustrate use of PL/pgSQL control
   flow structures:
CREATE TABLE db (a INT PRIMARY KEY, b TEXT);

CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
    LOOP
        -- first try to update the key
        UPDATE db SET b = data WHERE a = key;
        IF found THEN
            RETURN;
        END IF;
        -- not there, so try to insert the key
        -- if someone else inserts the same key concurrently,
        -- we could get a unique-key failure
        BEGIN
            INSERT INTO db(a,b) VALUES (key, data);
            RETURN;
        EXCEPTION WHEN unique_violation THEN
            -- Do nothing, and loop to try the UPDATE again.
        END;
    END LOOP;
END;
$$
LANGUAGE plpgsql;

SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');

   This coding assumes the unique_violation error is caused by the INSERT,
   and not by, say, an INSERT in a trigger function on the table. It might
   also misbehave if there is more than one unique index on the table,
   since it will retry the operation regardless of which index caused the
   error. More safety could be had by using the features discussed next to
   check that the trapped error was the one expected.

41.6.8.1. Obtaining Information about an Error #

   Exception handlers frequently need to identify the specific error that
   occurred. There are two ways to get information about the current
   exception in PL/pgSQL: special variables and the GET STACKED
   DIAGNOSTICS command.

   Within an exception handler, the special variable SQLSTATE contains the
   error code that corresponds to the exception that was raised (refer to
   Table A.1 for a list of possible error codes). The special variable
   SQLERRM contains the error message associated with the exception. These
   variables are undefined outside exception handlers.

   Within an exception handler, one may also retrieve information about
   the current exception by using the GET STACKED DIAGNOSTICS command,
   which has the form:
GET STACKED DIAGNOSTICS variable { = | := } item [ , ... ];

   Each item is a key word identifying a status value to be assigned to
   the specified variable (which should be of the right data type to
   receive it). The currently available status items are shown in
   Table 41.2.

   Table 41.2. Error Diagnostics Items
   Name Type Description
   RETURNED_SQLSTATE text the SQLSTATE error code of the exception
   COLUMN_NAME text the name of the column related to exception
   CONSTRAINT_NAME text the name of the constraint related to exception
   PG_DATATYPE_NAME text the name of the data type related to exception
   MESSAGE_TEXT text the text of the exception's primary message
   TABLE_NAME text the name of the table related to exception
   SCHEMA_NAME text the name of the schema related to exception
   PG_EXCEPTION_DETAIL text the text of the exception's detail message, if
   any
   PG_EXCEPTION_HINT text the text of the exception's hint message, if any
   PG_EXCEPTION_CONTEXT text line(s) of text describing the call stack at
   the time of the exception (see Section 41.6.9)

   If the exception did not set a value for an item, an empty string will
   be returned.

   Here is an example:
DECLARE
  text_var1 text;
  text_var2 text;
  text_var3 text;
BEGIN
  -- some processing which might cause an exception
  ...
EXCEPTION WHEN OTHERS THEN
  GET STACKED DIAGNOSTICS text_var1 = MESSAGE_TEXT,
                          text_var2 = PG_EXCEPTION_DETAIL,
                          text_var3 = PG_EXCEPTION_HINT;
END;

41.6.9. Obtaining Execution Location Information #

   The GET DIAGNOSTICS command, previously described in Section 41.5.5,
   retrieves information about current execution state (whereas the GET
   STACKED DIAGNOSTICS command discussed above reports information about
   the execution state as of a previous error). Its PG_CONTEXT status item
   is useful for identifying the current execution location. PG_CONTEXT
   returns a text string with line(s) of text describing the call stack.
   The first line refers to the current function and currently executing
   GET DIAGNOSTICS command. The second and any subsequent lines refer to
   calling functions further up the call stack. For example:
CREATE OR REPLACE FUNCTION outer_func() RETURNS integer AS $$
BEGIN
  RETURN inner_func();
END;
$$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION inner_func() RETURNS integer AS $$
DECLARE
  stack text;
BEGIN
  GET DIAGNOSTICS stack = PG_CONTEXT;
  RAISE NOTICE E'--- Call Stack ---\n%', stack;
  RETURN 1;
END;
$$ LANGUAGE plpgsql;

SELECT outer_func();

NOTICE:  --- Call Stack ---
PL/pgSQL function inner_func() line 5 at GET DIAGNOSTICS
PL/pgSQL function outer_func() line 3 at RETURN
CONTEXT:  PL/pgSQL function outer_func() line 3 at RETURN
 outer_func
 ------------
           1
(1 row)

   GET STACKED DIAGNOSTICS ... PG_EXCEPTION_CONTEXT returns the same sort
   of stack trace, but describing the location at which an error was
   detected, rather than the current location.