Plantuner - enable planner hints

plantuner is a contribution module for PostgreSQL 9.5+, which enable planner hints.

All work was done by Teodor Sigaev ([email protected]) and Oleg Bartunov ([email protected]).

Sponsor: Nomao project (http://www.nomao.com)

Motivation

Whether somebody think it's bad or not, but sometime it's very interesting to be able to control planner (provide hints, which tells optimizer to ignore its algorithm in part), which is currently impossible in POstgreSQL. Oracle, for example, has over 120 hints, SQL Server also provides hints.

This first version of plantuner provides a possibility to hide specified indexes from PostgreSQL planner, so it will not use them.

There are many situation, when developer want to temporarily disable specific index(es), without dropping them, or to instruct planner to use specific index.

Next, for some workload PostgreSQL could be too pessimistic for newly created tables and assumes much more rows in table than it actually has. If plantuner.fix_empty_table GUC variable is set to true then module will set to zero number of pages/tuples of table which hasn't blocks in file.

Installation

Get the latest source code.

export USE_PGXS=1
gmake
gmake install
gmake installcheck

Syntax

• plantuner.forbid_index (deprecated)

• plantuner.disable_index

  List of indexes invisible to planner

• plantuner.enable_index

  List of indexes visible to planner even they are hided by plantuner.disable_index.

Usage

To enable the module you can either load shared library plantuner in 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)