Database

pg_plan_filter: Restrict Total Cost

pg_plan_filter is Postgres 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:

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load 'plan_filter';

or for all connections:

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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 modify data, for example, 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:

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

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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.

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load 'plan_filter';set plan_filter.statement_cost_limit = 50; -- between 2.49 and 135.0select * from book where id = 1; id----  1(1 row)-- SUCCESS
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select * from book;ERROR:  plan cost limit exceededHINT:  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

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