---
id: 'pg_plan_filter'
title: 'pg_plan_filter: Restrict Total Cost'
description: 'Block queries over a total cost limit'
---

[`pg_plan_filter`](https://github.com/pgexperts/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:

{/* prettier-ignore */}
```sql
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.

{/* prettier-ignore */}
```sql
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.

{/* prettier-ignore */}
```sql
set plan_filter.statement_cost_limit = 50; -- between 2.49 and 135.0

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

{/* prettier-ignore */}
```sql
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](https://github.com/pgexperts/pg_plan_filter)
