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
.
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
RETURN NEXT
and RETURN QUERY
RETURN 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 sometype
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
when there is just one output parameter of type
sometype
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);
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.
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.
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 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 $$ DECLARE myvar int := 5; BEGIN CALL triple(myvar); RAISE NOTICE 'myvar = %', myvar; -- prints 15 END; $$;
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-THEN
IFboolean-expression
THENstatements
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;
IF-THEN-ELSE
IFboolean-expression
THENstatements
ELSEstatements
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;
IF-THEN-ELSIF
IFboolean-expression
THENstatements
[ ELSIFboolean-expression
THENstatements
[ ELSIFboolean-expression
THENstatements
... ] ] [ 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.
CASE
CASEsearch-expression
WHENexpression
[,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;
CASE
CASE WHENboolean-expression
THENstatements
[ WHENboolean-expression
THENstatements
... ] [ ELSEstatements
] 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
>> ] LOOPstatements
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.
EXIT
EXIT [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;
CONTINUE
CONTINUE [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-expression
LOOPstatements
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;
FOR
(Integer Variant)[ <<label
>> ] FORname
IN [ REVERSE ]expression
..expression
[ BYexpression
] LOOPstatements
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
.
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
>> ] FORtarget
INquery
LOOPstatements
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 $$ DECLARE mviews RECORD; BEGIN RAISE NOTICE 'Refreshing all materialized views...'; FOR mviews IN SELECT n.nspname AS mv_schema, c.relname AS mv_name, pg_catalog.pg_get_userbyid(c.relowner) AS owner FROM pg_catalog.pg_class c LEFT JOIN pg_catalog.pg_namespace n ON (n.oid = c.relnamespace) WHERE c.relkind = 'm' ORDER BY 1 LOOP -- Now "mviews" has one record with information about the materialized view RAISE NOTICE 'Refreshing materialized view %.% (owner: %)...', quote_ident(mviews.mv_schema), quote_ident(mviews.mv_name), quote_ident(mviews.owner); EXECUTE format('REFRESH MATERIALIZED VIEW %I.%I', mviews.mv_schema, mviews.mv_name); END LOOP; RAISE NOTICE 'Done refreshing materialized views.'; RETURN 1; END; $$ 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 42.11.1절 and 42.11.2절.
The FOR-IN-EXECUTE
statement is another way to iterate over
rows:
[ <<label
>> ] FORtarget
IN EXECUTEtext_expression
[ USINGexpression
[, ... ] ] LOOPstatements
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 42.7.4절.
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 ARRAYexpression
LOOPstatements
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}
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
] BEGINstatements
EXCEPTION 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 부록 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.
예 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 표 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 표 42.2.
표 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 42.6.9절) |
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 42.5.5절, 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.