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A PostgreSQL extension for collecting statistics about predicates, helping find what indices are missing


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pg_qualstats is a PostgreSQL extension keeping statistics on predicates found in WHERE statements and JOIN clauses.

This is useful if you want to be able to analyze what are the most-often executed quals (predicates) on your database. The powa project makes use of this to provide advances index suggestions.

It also allows you to identify correlated columns, by identifying which columns are most frequently queried together.

The extension works by looking for known patterns in queries. Currently, this includes:

  • Binary OpExpr where at least one side is a column from a table. Whenever possible, the predicate will be swaped so that CONST OP VAR expressions are turned into VAR COMMUTED_OP CONST. AND and OR expression members are counted as separate entries. Ex: WHERE column1 = 2, WHERE column1 = column2, WHERE 3 = column3

  • ScalarArrayOpExpr where the left side is a VAR, and the right side is an array constant. Those will be counted one time per element in the array. Ex: WHERE column1 IN (2, 3) will be counted as 2 occurences for the (column1, '=') operator pair

  • BooleanTest where the expression is a simple boolean column reference Ex: WHERE column1 IS TRUE Please not that clauses like WHERE columns1, WHERE NOT column1 won't be processed by pg_qualstats (yet)

This extension also saves the first query text, as-is, for each distinct queryid executed, with a limit of pg_qualstats.max entries.

Please not that the gathered data are not saved when the PostgreSQL server is restarted.


  • Compatible with PostgreSQL 9.4 or later
  • Needs postgresql header files
  • sudo make install
  • Add pg_qualstats to the shared preload libraries:
   shared_preload_libraries = 'pg_qualstats'


The following GUCs can be configured, in postgresql.conf:

  • pg_qualstats.enabled (boolean, default true): whether or not pg_qualstats should be enabled
  • pg_qualstats.track_constants (bolean, default true): whether or not pg_qualstats should keep track of each constant value individually. Disabling this GUC will considerably reduce the number of entries necessary to keep track of predicates.
  • pg_qualstats.max: the maximum number of predicated and query text tracked (defaults to 1000)
  • pg_qualstats.resolve_oids (boolean, default false): whether or not pg_qualstats should resolve oids at query time, or juste store the oids. Enabling this parameter makes the data analysis much more easy, since a connection to the database where the query was executed won't be necessary, but it will eat much more space (624 bytes per entry instead of 176). Additionnaly, this will require some catalog lookups, which aren't free.
  • pg_qualstats.track_pg_catalog (boolean, default false): whether or not pg_qualstats should compute predicates on object in pg_catalog schema.
  • pg_qualstats.sample_rate (double, default -1): the fraction of queries that should be sampled. For example, 0.1 means that only one out of ten queries will be sampled. The default (-1) means automatic, and results in a value of 1 / max_connections, so that statiscally, concurrency issues will be rare.

Updating the extension

Note that as all extensions configured in shared_preload_libraries, most of the changes are only applied once PostgreSQL is restarted with the new shared library version. The extension objects themselves only provides SQL wrappers to access internal data structures.

Since version 2.0.4, an upgrade script is provided, allowing to upgade from the previous version only. If you want to upgrade the extension across multiple versions, or from a version older than 2.0.3, you will need top drop and recreate the extension to get the latest version.


  • Create the extension in any database:
   CREATE EXTENSION pg_qualstats;


The extension defines the following functions:

  • pg_qualstats: returns the counts for every qualifier, identified by the expression hash. This hash identifies each expression.

    • userid: oid of the user who executed the query.
    • dbid: oid of the database in which the query has been executed.
    • lrelid, lattnum: oid of the relation and attribute number of the VAR on the left hand side, if any.
    • opno: oid of the operator used in the expression
    • rrelid, rattnum: oid of the relation and attribute number of the VAR on the right hand side, if any.
    • qualid: normalized identifier of the parent "AND" expression, if any. This identifier is computed excluding the constants. This is useful for identifying predicates which are used together.
    • uniquequalid: unique identifier of the parent "AND" expression, if any. This identifier is computed including the constants.
    • qualnodeid: normalized identifier of this simple predicate. This identifier is computed excluding the constants.
    • uniquequalnodeid: unique identifier of this simple predicate. This identifier is computed including the constats.
    • occurences: number of time this predicate has been invoked, ie. number of related query execution.
    • execution_count: number of time this predicate has been executed, ie. number of rows it processed.
    • nbfiltered: number of tuples this predicate discarded.
    • constant_position: location of the constant in the original query string, as reported by the parser.
    • queryid: if pg_stats_statements is installed, the queryid identifying this query, otherwise NULL.
    • constvalue: a string representation of the right-hand side constant, if any, truncated to 80 bytes. Require to be superuser or member of pg_read_all_stats (since PostgreSQL 10), "" will be showed instead.
    • eval_type: evaluation type. 'f' for a predicate evaluated after a scan or 'i' for an index predicate.


ro=# select * from pg_qualstats;
 userid │ dbid  │ lrelid │ lattnum │ opno │ rrelid │ rattnum │ qualid │ uniquequalid │ qualnodeid │ uniquequalnodeid │ occurences │ execution_count │ nbfiltered │ constant_position │ queryid │   constvalue   │ eval_type
     10 │ 16384 │  16385 │       2 │   98 │ <NULL> │  <NULL> │ <NULL> │       <NULL> │  115075651 │       1858640877 │          1 │          100000 │      99999 │                29 │  <NULL> │ 'line 1'::text │ f
     10 │ 16384 │  16391 │       2 │   98 │  16385 │       2 │ <NULL> │       <NULL> │  497379130 │        497379130 │          1 │               0 │          0 │            <NULL> │  <NULL> │                │ f
  • pg_qualstats_index_advisor(min_filter, min_selectivity, forbidden_am): Perform a global index suggestion. By default, only predicates filtering at least 1000 rows and 30% of the rows in average will be considered, but this can be passed as parameter. You can also provide an array of index access method if you want to avoid some. For instance, on PostgreSQL 9.6 and prior, hash indexes will be ignored as those weren't crash safe yet.


  FROM json_array_elements(
    pg_qualstats_index_advisor(min_filter => 50)->'indexes') v
  ORDER BY v::text COLLATE "C";
 "CREATE INDEX ON public.adv USING btree (id1)"
 "CREATE INDEX ON public.adv USING btree (val, id1, id2, id3)"
 "CREATE INDEX ON public.pgqs USING btree (id)"
(3 rows)

  FROM json_array_elements(
    pg_qualstats_index_advisor(min_filter => 50)->'unoptimised') v
  ORDER BY v::text COLLATE "C";
 "adv.val ~~* ?"
(1 row)
  • pg_qualstats_deparse_qual: format a stored predicate in the form tablename.columname operatorname ?. This is mostly for the global index advisor.
  • pg_qualstats_get_idx_col: for the given predicate, retrieve the underlying column name and all the possible operator class. This is mostly for the global index advisor.
  • pg_qualstats_get_qualnode_rel: for the given predicate, return the underlying table, fully qualified. This is mostly for the global index advisor
  • pg_qualstats_example_queries: return all the stored query texts.
  • pg_qualstats_example_query: return the stored query text for the given queryid if any, otherwise NULL.
  • pg_qualstats_names: return all the stored query texts.
  • pg_qualstats_reset: reset the internal counters and forget about every encountered qual.


In addition to that, the extension defines some views on top of the pg_qualstats function:

  • pg_qualstats: filters calls to pg_qualstats() by the current database.

  • pg_qualstats_pretty: performs the appropriate joins to display a readable aggregated form for every attribute from the pg_qualstats view


ro=# select * from pg_qualstats_pretty;
 left_schema |    left_table    | left_column |   operator   | right_schema | right_table | right_column | occurences | execution_count | nbfiltered
 public      | pgbench_accounts | aid         | pg_catalog.= |              |             |              |          5 |         5000000 |    4999995
 public      | pgbench_tellers  | tid         | pg_catalog.= |              |             |              |         10 |        10000000 |    9999990
 public      | pgbench_branches | bid         | pg_catalog.= |              |             |              |         10 |         2000000 |    1999990
 public      | t1               | id          | pg_catalog.= | public       | t2          | id_t1        |          1 |           10000 |       9999
  • pg_qualstats_all: sums the counts for each attribute / operator pair, regardless of its position as an operand (LEFT or RIGHT), grouping together attributes used in AND clauses.


ro=# select * from pg_qualstats_all;
 dbid  | relid | userid | queryid | attnums | opno | qualid | occurences | execution_count | nbfiltered | qualnodeid
 16384 | 16385 |     10 |         | {2}     |   98 |        |          1 |          100000 |      99999 |  115075651
 16384 | 16391 |     10 |         | {2}     |   98 |        |          2 |               0 |          0 |  497379130
  • pg_qualstats_by_query: returns only predicates of the form VAR OPERATOR CONSTANT, aggregated by queryid.