## F.38. plantuner

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

### F.38.1. 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 PostgreSQL planner, so it will not use them. For some workloads, PostgreSQL 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.38.2. 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.38.3. 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.38.4. 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).