Get query plan for SQL statement nested in a PL/pgSQL function

Is it possible to get the query plan for an SQL statement executed inside a user defined function (UDF)? Like with using EXPLAIN in pgAdmin or psql as client. I see the UDF abstracted away into a single operation F() in pgAdmin.

Currently, I pull out statements and run them manually. But this isn't going to cut it for large queries.

For example, consider the UDF below. It can print its dynamically generated query string. But that can't be run separately as it depends on a temporary table created in the local context.

```
CREATE OR REPLACE FUNCTION get_paginated_search_results(
forum_id_ INTEGER,
query_ CHARACTER VARYING,
from_date_ TIMESTAMP WITHOUT TIME ZONE DEFAULT NULL,
to_date_ TIMESTAMP WITHOUT TIME ZONE DEFAULT NULL,
in_categories_ INTEGER[] DEFAULT '{}')
RETURNS SETOF post_result_entry AS $$
DECLARE
join_string CHARACTER VARYING := ' ';
from_where_date CHARACTER VARYING := ' ';
to_where_date CHARACTER VARYING := ' ';
query_string_ CHARACTER VARYING := ' ';
BEGIN
IF NOT from_date_ IS NULL THEN
from_where_date := ' AND fp.posted_at > ''' || from_date_ || '''';
END IF;

IF NOT to_date_ IS NULL THEN
to_where_date := ' AND fp.posted_at < ''' || to_date_ || '''';
END IF;

CREATE LOCAL TEMP TABLE un_cat(id) ON COMMIT DROP AS (select * from unnest(in_categories_)) ;

if in_categories_ != '{}' THEN
join_string := ' INNER JOIN forum_topics ft ON fp.topic_id = ft.id ' ||
' INNER JOIN un_cat uc ON uc.id = ft.category_id ' ;
END IF;

query_string_ := '
SELECT index,posted_at,post_text,name,join_date,quotes
FROM forum_posts fp
INNER JOIN forum_user fu ON
fu.forum_id = fp.forum_id AND fu.id = fp.user_id' ||
join_string
||
'WHERE fu.forum_id = ' || forum_id_ || ' AND
to_tsvector(''english'',fp.post_text) @@ to_tsquery(''english'','''|| query_||''')' ||
from_where_date ||
to_where_date
||';';

RAISE NOTICE

I addition to rfusca's advice: SQL statements inside PL/pgSQL functions (or any function that is not inlined) are considered nested statements. Set the parameter auto_explain.log_nested_statements to include those.

You don't need CREATE EXTENSION like for most extensions. Just LOAD it dynamically into your session.

You must be superuser for this:
LOAD 'auto_explain';
SET auto_explain.log_nested_statements = ON; -- statements inside functions
SET auto_explain.log_min_duration = 1; -- exclude very fast queries taking

Set auto_explain.log_min_duration to some other value than -1 to log any plans at all. The manual:

auto_explain.log_min_duration is the minimum statement execution
time, in milliseconds, that will cause the statement's plan to be
logged. Setting this to 0 logs all plans. -1 (the default)
disables logging of plans. For example, if you set it to 250ms then
all statements that run 250ms or longer will be logged. Only
superusers can change this setting.

May produce a lot of log output.

Log messages are written to log files with default settings. To get them in the client directly (works at least in psql):

SET client_min_messages TO log;

See:

• Getting the query plan for statements inside a stored procedure in PostgreSQL

Depesz wrote a blog article about it when it was introduced with PostgreSQL 8.4.

StackExchange Database Administrators Q#23355, answer score: 24