HypoPG

HypoPG is a PostgreSQL extension adding support for hypothetical indexes.

An hypothetical -- or virtual -- index is an index that doesn't really exists, andthus doesn't cost CPU, disk or any resource to create. They're useful to knowif specific indexes can increase performance for problematic queries, sinceyou can know if PostgreSQL will use these indexes or not without having tospend resources to create them.

For more thorough informations, please consult the officialdocumentation.

For other general information, you can also consult this blogpost.

Installation

• Compatible with PostgreSQL 9.2 and above
• Needs PostgreSQL header files
• Decompress the tarball
• sudo make install
• In every needed database: CREATE EXTENSION hypopg;

Updating the extension

Note that hypopg doesn't provide extension upgrade scripts, as there's nodata saved in any of the objects created. Therefore, you need to first dropthe extension then create it again to get the new version.

Usage

NOTE: The hypothetical indexes are contained in a single backend. Therefore,if you add multiple hypothetical indexes, concurrent connections doingEXPLAIN won't be bothered by your hypothetical indexes.

Assuming a simple test case:

rjuju=# CREATE TABLE hypo AS SELECT id, 'line ' || id AS val FROM generate_series(1,10000) id;
rjuju=# EXPLAIN SELECT * FROM hypo WHERE id = 1;
                      QUERY PLAN
-------------------------------------------------------
 Seq Scan on hypo  (cost=0.00..180.00 rows=1 width=13)
   Filter: (id = 1)
(2 rows)

The easiest way to create an hypothetical index is to use thehypopg_create_index functions with a regular CREATE INDEX statement as arg.

For instance:

rjuju=# SELECT * FROM hypopg_create_index('CREATE INDEX ON hypo (id)');

NOTE: Some information from the CREATE INDEX statement will be ignored, such asthe index name if provided. Some of the ignored information will be handled ina future release.

You can check the available hypothetical indexes in your own backend:

rjuju=# SELECT * FROM hypopg_list_indexes();
 indexrelid |                 indexname                 | nspname | relname | amname
 -----------+-------------------------------------------+---------+---------+--------
     205101 | <41072>btree_hypo_id                      | public  | hypo    | btree

If you need more technical information on the hypothetical indexes, thehypopg() function will return the hypothetical indexes in a similar way aspg_index system catalog.

And now, let's see if your previous EXPLAIN statement would use such an index:

rjuju=# EXPLAIN SELECT * FROM hypo WHERE id = 1;
                                     QUERY PLAN
------------------------------------------------------------------------------------
 Index Scan using <41072>hypo_btree_hypo_id on hypo  (cost=0.29..8.30 rows=1 width=13)
   Index Cond: (id = 1)
(2 rows)

Of course, only EXPLAIN without ANALYZE will use hypothetical indexes:

rjuju=# EXPLAIN ANALYZE SELECT * FROM hypo WHERE id = 1;
                                           QUERY PLAN
-------------------------------------------------------------------------------------------------
 Seq Scan on hypo  (cost=0.00..180.00 rows=1 width=13) (actual time=0.036..6.072 rows=1 loops=1)
   Filter: (id = 1)
   Rows Removed by Filter: 9999
 Planning time: 0.109 ms
 Execution time: 6.113 ms
(5 rows)

To remove your backend's hypothetical indexes, you can use the functionhypopg_drop_index(indexrelid) with the OID that the hypopg_list_indexes()function returns and call hypopg_reset() to remove all at once, or just closeyour current connection.