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.
     There are two commands available that allow you to return data
     from a function: RETURN and RETURN
     NEXT.
    
RETURNRETURN 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
RETURN NEXT and RETURN QUERYRETURN NEXTexpression; RETURN QUERYquery; RETURN QUERY EXECUTEcommand-string[ USINGexpression[, ... ] ];
      When a PL/pgSQL function is declared to return
      SETOF , the procedure
      to follow is slightly different.  In that case, the individual
      items to return are specified by a sequence of sometypeRETURN
      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 
      when there is just one output parameter of type
      sometypesometype, 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);
       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.
      
     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
IF-THENIFboolean-expressionTHENstatementsEND 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;
IF-THEN-ELSEIFboolean-expressionTHENstatementsELSEstatementsEND 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;
IF-THEN-ELSIFIFboolean-expressionTHENstatements[ ELSIFboolean-expressionTHENstatements[ ELSIFboolean-expressionTHENstatements...]] [ ELSEstatements] 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.
       
CASECASEsearch-expressionWHENexpression[,expression[ ... ]] THENstatements[ WHENexpression[,expression[ ... ]] THENstatements... ] [ ELSEstatements] 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;
CASECASE
    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.
      
     With the LOOP, EXIT,
     CONTINUE, WHILE, FOR,
     and FOREACH statements, you can arrange for your
     PL/pgSQL function to repeat a series of commands.
    
LOOP[ <<label>> ] LOOPstatementsEND 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.
     
EXITEXIT [label] [ WHENboolean-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;
CONTINUECONTINUE [label] [ WHENboolean-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;
WHILE[ <<label>> ] WHILEboolean-expressionLOOPstatementsEND 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;
FOR (Integer Variant)[ <<label>> ] FORnameIN [ REVERSE ]expression..expression[ BYexpression] LOOPstatementsEND 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.
       
     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>> ] FORtargetINqueryLOOPstatementsEND 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 $$
DECLARE
    mviews RECORD;
BEGIN
    RAISE NOTICE 'Refreshing all materialized views...';
    FOR mviews IN
       SELECT n.nspname AS mv_schema,
              c.relname AS mv_name,
              pg_catalog.pg_get_userbyid(c.relowner) AS owner
         FROM pg_catalog.pg_class c
    LEFT JOIN pg_catalog.pg_namespace n ON (n.oid = c.relnamespace)
        WHERE c.relkind = 'm'
     ORDER BY 1
    LOOP
        -- Now "mviews" has one record with information about the materialized view
        RAISE NOTICE 'Refreshing materialized view %.% (owner: %)...',
                     quote_ident(mviews.mv_schema),
                     quote_ident(mviews.mv_name),
                     quote_ident(mviews.owner);
        EXECUTE format('REFRESH MATERIALIZED VIEW %I.%I', mviews.mv_schema, mviews.mv_name);
    END LOOP;
    RAISE NOTICE 'Done refreshing materialized views.';
    RETURN 1;
END;
$$ 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, or
     DELETE with a RETURNING clause.  Some utility
     commands such as EXPLAIN will work too.
    
PL/pgSQL variables are substituted into the query text, and the query plan is cached for possible re-use, as discussed in detail in Section 42.10.1 and Section 42.10.2.
     The FOR-IN-EXECUTE statement is another way to iterate over
     rows:
[ <<label>> ] FORtargetIN EXECUTEtext_expression[ USINGexpression[, ... ] ] LOOPstatementsEND 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 42.7.4.
     The FOREACH loop is much like a FOR loop,
     but instead of iterating through the rows returned by a 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>> ] FOREACHtarget[ SLICEnumber] IN ARRAYexpressionLOOPstatementsEND 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}
     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>> ] [ DECLAREdeclarations] BEGINstatementsEXCEPTION WHENcondition[ ORcondition... ] THENhandler_statements[ WHENcondition[ ORcondition... ] THENhandler_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.
    
      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 42.2. 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.
    
     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 DIAGNOSTICSvariable{ = | := }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 42.2.
    
Table 42.2. 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 42.6.7) | 
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;
    The GET DIAGNOSTICS command, previously described
    in Section 42.5.5, 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.