pg_plan_filter: Restrict Total Cost

pg_plan_filter is PostgreSQL extension to block execution of statements where query planner's estimate of the total cost exceeds a threshold. This is intended to give database administrators a way to restrict the contribution an individual query has on database load.

Enable the extension#

pg_plan_filter can be enabled on a per connection basis:

load 'plan_filter';

or for all connections:

alter database some_db set session_preload_libraries = 'plan_filter';

API#

plan_filter.statement_cost_limit: restricts the maximum total cost for executed statements plan_filter.limit_select_only: restricts to select statements

Note that limit_select_only = true is not the same as read-only because select statements may modfiy data e.g. through a function call.

Example#

To demonstrate total cost filtering, we'll compare how plan_filter.statement_cost_limit treats queries that are under and over its cost limit. First, we set up a table with some data:

create table book(
  id int primary key
);
-- CREATE TABLE

insert into book(id) select * from generate_series(1, 10000);
-- INSERT 0 10000

Next, we can review the explain plans for a single record select, and a whole table select.

explain select * from book where id =1;
                                QUERY PLAN
---------------------------------------------------------------------------
 Index Only Scan using book_pkey on book  (cost=0.28..2.49 rows=1 width=4)
   Index Cond: (id = 1)
(2 rows)

explain select * from book;
                       QUERY PLAN
---------------------------------------------------------
 Seq Scan on book  (cost=0.00..135.00 rows=10000 width=4)
(1 row)

Now we can choose a statement_cost_filter value between the total cost for the single select (2.49) and the whole table select (135.0) so one statement will succeed and one will fail.

load 'plan_filter';
set plan_filter.statement_cost_limit = 50; -- between 2.49 and 135.0

select * from book where id = 1;
 id
----
  1
(1 row)
-- SUCCESS

select * from book;

ERROR:  plan cost limit exceeded
HINT:  The plan for your query shows that it would probably have an excessive run time. This may be due to a logic error in the SQL, or it maybe just a very costly query. Rewrite your query or increase the configuration parameter "plan_filter.statement_cost_limit".
-- FAILURE

Resources#

Official pg_plan_filter documentation

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