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When evaluating a query with a multi-column GROUP BY clause using sort, the cost may be heavily dependent on the order in which the keys are compared when building the groups. Grouping does not imply any ordering, so we're allowed to compare the keys in arbitrary order, and a Hash Agg leverages this. But for Group Agg, we simply compared keys in the order as specified in the query. This commit explores alternative ordering of the keys, trying to find a cheaper one. In principle, we might generate grouping paths for all permutations of the keys, and leave the rest to the optimizer. But that might get very expensive, so we try to pick only a couple interesting orderings based on both local and global information. When planning the grouping path, we explore statistics (number of distinct values, cost of the comparison function) for the keys and reorder them to minimize comparison costs. Intuitively, it may be better to perform more expensive comparisons (for complex data types etc.) last, because maybe the cheaper comparisons will be enough. Similarly, the higher the cardinality of a key, the lower the probability we’ll need to compare more keys. The patch generates and costs various orderings, picking the cheapest ones. The ordering of group keys may interact with other parts of the query, some of which may not be known while planning the grouping. E.g. there may be an explicit ORDER BY clause, or some other ordering-dependent operation, higher up in the query, and using the same ordering may allow using either incremental sort or even eliminate the sort entirely. The patch generates orderings and picks those minimizing the comparison cost (for various pathkeys), and then adds orderings that might be useful for operations higher up in the plan (ORDER BY, etc.). Finally, it always keeps the ordering specified in the query, on the assumption the user might have additional insights. This introduces a new GUC enable_group_by_reordering, so that the optimization may be disabled if needed. The original patch was proposed by Teodor Sigaev, and later improved and reworked by Dmitry Dolgov. Reviews by a number of people, including me, Andrey Lepikhov, Claudio Freire, Ibrar Ahmed and Zhihong Yu. Author: Dmitry Dolgov, Teodor Sigaev, Tomas Vondra Reviewed-by: Tomas Vondra, Andrey Lepikhov, Claudio Freire, Ibrar Ahmed, Zhihong Yu Discussion: https://postgr.es/m/7c79e6a5-8597-74e8-0671-1c39d124c9d6%40sigaev.ru Discussion: https://postgr.es/m/CA%2Bq6zcW_4o2NC0zutLkOJPsFt80megSpX_dVRo6GK9PC-Jx_Ag%40mail.gmail.com |
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| .. | ||
| adminpack | ||
| amcheck | ||
| auth_delay | ||
| auto_explain | ||
| basebackup_to_shell | ||
| basic_archive | ||
| bloom | ||
| bool_plperl | ||
| btree_gin | ||
| btree_gist | ||
| citext | ||
| cube | ||
| dblink | ||
| dict_int | ||
| dict_xsyn | ||
| earthdistance | ||
| file_fdw | ||
| fuzzystrmatch | ||
| hstore | ||
| hstore_plperl | ||
| hstore_plpython | ||
| intagg | ||
| intarray | ||
| isn | ||
| jsonb_plperl | ||
| jsonb_plpython | ||
| lo | ||
| ltree | ||
| ltree_plpython | ||
| oid2name | ||
| old_snapshot | ||
| pageinspect | ||
| passwordcheck | ||
| pg_buffercache | ||
| pg_freespacemap | ||
| pg_prewarm | ||
| pg_stat_statements | ||
| pg_surgery | ||
| pg_trgm | ||
| pg_visibility | ||
| pgcrypto | ||
| pgrowlocks | ||
| pgstattuple | ||
| postgres_fdw | ||
| seg | ||
| sepgsql | ||
| spi | ||
| sslinfo | ||
| start-scripts | ||
| tablefunc | ||
| tcn | ||
| test_decoding | ||
| tsm_system_rows | ||
| tsm_system_time | ||
| unaccent | ||
| uuid-ossp | ||
| vacuumlo | ||
| xml2 | ||
| contrib-global.mk | ||
| Makefile | ||
| README | ||
The PostgreSQL contrib tree
---------------------------
This subtree contains porting tools, analysis utilities, and plug-in
features that are not part of the core PostgreSQL system, mainly
because they address a limited audience or are too experimental to be
part of the main source tree. This does not preclude their
usefulness.
User documentation for each module appears in the main SGML
documentation.
When building from the source distribution, these modules are not
built automatically, unless you build the "world" target. You can
also build and install them all by running "make all" and "make
install" in this directory; or to build and install just one selected
module, do the same in that module's subdirectory.
Some directories supply new user-defined functions, operators, or
types. To make use of one of these modules, after you have installed
the code you need to register the new SQL objects in the database
system by executing a CREATE EXTENSION command. In a fresh database,
you can simply do
CREATE EXTENSION module_name;
See the PostgreSQL documentation for more information about this
procedure.