F.44. plantuner

The plantuner module provides hints for the planner that can disable or enable indexes for query execution.

F.44.1. Motivation

In some cases, it may be required to control the planner by providing hints that make the optimizer ignore some parts of its algorithm. There are many situations when a developer may want to temporarily disable specific index(es), without dropping them, or to instruct the planner to use a specific index.

This version of plantuner provides a possibility to hide the specified indexes from Postgres Pro planner, so it will not use them. For some workloads, Postgres Pro could be too pessimistic about newly created tables and assume that there are much more rows in a table than it actually has. If the plantuner.fix_empty_table GUC variable is set to true, plantuner sets to zero the number of pages/tuples of the table that has no blocks in a file.

F.44.2. GUC Variables

plantuner.disable_index — list of indexes invisible to planner.

plantuner.enable_index — list of indexes visible to planner even if they are hidden by plantuner.disable_index.

F.44.3. Example

To enable the module, you can either load plantuner shared library in a psql session or specify shared_preload_libraries option in postgresql.conf.

=# LOAD 'plantuner';
=# create table test(id int);
=# create index id_idx on test(id);
=# create index id_idx2 on test(id);
=# \d test
     Table "public.test"
 Column |  Type   | Modifiers
--------+---------+-----------
 id     | integer |
Indexes:
    "id_idx" btree (id)
    "id_idx2" btree (id)
=# explain select id from test where id=1;
                              QUERY PLAN
-----------------------------------------------------------------------
 Bitmap Heap Scan on test  (cost=4.34..15.03 rows=12 width=4)
   Recheck Cond: (id = 1)
   ->  Bitmap Index Scan on id_idx2  (cost=0.00..4.34 rows=12 width=0)
         Index Cond: (id = 1)
(4 rows)
=# set enable_seqscan=off;
=# set plantuner.disable_index='id_idx2';
=# explain select id from test where id=1;
                              QUERY PLAN
----------------------------------------------------------------------
 Bitmap Heap Scan on test  (cost=4.34..15.03 rows=12 width=4)
   Recheck Cond: (id = 1)
   ->  Bitmap Index Scan on id_idx  (cost=0.00..4.34 rows=12 width=0)
         Index Cond: (id = 1)
(4 rows)
=# set plantuner.disable_index='id_idx2,id_idx';
=# explain select id from test where id=1;
                               QUERY PLAN
-------------------------------------------------------------------------
 Seq Scan on test  (cost=10000000000.00..10000000040.00 rows=12 width=4)
   Filter: (id = 1)
(2 rows)
=# set plantuner.enable_index='id_idx';
=# explain select id from test where id=1;
                              QUERY PLAN
-----------------------------------------------------------------------
 Bitmap Heap Scan on test  (cost=4.34..15.03 rows=12 width=4)
   Recheck Cond: (id = 1)
   ->  Bitmap Index Scan on id_idx  (cost=0.00..4.34 rows=12 width=0)
         Index Cond: (id = 1)
(4 rows)
      

F.44.4. Authors

All work was done by Teodor Sigaev (teodor@sigaev.ru) and Oleg Bartunov (oleg@sai.msu.su).

The work sponsored by Nomao project (http://www.nomao.com).