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

The extension is already enabled by default via shared_preload_libraries setting.

You can follow the instructions below.

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(
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  id int primary key
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);
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-- CREATE TABLE
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insert into book(id) select * from generate_series(1, 10000);
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-- 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;
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                                QUERY PLAN
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---------------------------------------------------------------------------
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 Index Only Scan using book_pkey on book  (cost=0.28..2.49 rows=1 width=4)
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   Index Cond: (id = 1)
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(2 rows)
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explain select * from book;
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                       QUERY PLAN
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---------------------------------------------------------
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 Seq Scan on book  (cost=0.00..135.00 rows=10000 width=4)
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(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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set plan_filter.statement_cost_limit = 50; -- between 2.49 and 135.0
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select * from book where id = 1;
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 id
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----
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  1
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(1 row)
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-- SUCCESS
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select * from book;
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ERROR:  plan cost limit exceeded
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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".
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-- FAILURE

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