pg_qualstats

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

The goal of this extension is to allow the DBA to answer some specific questions, whose answers are quite hard to come by:

  • what is the set of queries using this column ?
  • what are the values this where clause is most often using ?
  • do I have some significant skew in the distribution of the number of returned rows if use some value instead of one another ?
  • which columns are often used together in a WHERE clause ?

Where is it used in powa-web ?

If the extension is available, you should see a “list of quals” table on the query page, as well as explain plans for your query and a list of index suggestions:

../_images/pg_qualstats.png

From this list, you can then go on to the per-qual page.

Installation

As seen in Quickstart, the PostgreSQL development packages should be available.

First, download and extract the latest release of pg_qualstats:

wget https://github.com/powa-team/pg_qualstats/archive/1.0.2.tar.gz -O pg_qualstats-1.0.2.tar.gz
tar zxvf pg_qualstats-1.0.2.tar.gz
cd pg_qualstats-1.0.2

Then, compile the extension:

make

Then install the compiled package:

make install

Then you just have to declare the extension in the postgresql.conf file, like this :

shared_preload_libraries = 'pg_stat_statements,pg_qualstats'

Restart the PostgreSQL server to reload the libraries.

Connect to the server as a superuser and type:

CREATE EXTENSION pg_qualstats;

Using with PoWA

If you want PoWA to handle this extension, you have to connect as a superuser on the database where you installed PoWA, and type:

SELECT powa_qualstats_register();

Configuration

The following configuration parameters are available, in postgresql.conf:

pg_qualstats.enabled:
Defaults to true. Enable pg_qualstats. Can be useful if you want to enable / disable it without restarting the server.
pg_qualstats.max:
Defaults to 1000. Number of entries to keep. As a rule of thumb, you should keep at least pg_stat_statements.max entries if pg_qualstats.track_constants is disabled, else it should be roughly equal to the number of queries executed during powa.frequency interval of time.
pg_qualstats.track_pg_catalog:
Defaults to false. Determine if predicates on pg_catalog tables should be tracked too.
pg_qualstats.resolve_oids:
Defaults to false. Determine if during predicates collection, the actual name of the objects should be stored alongside their OIDs. The overhead is quite non-negligible, since each entry will occupy 616 bytes instead of 168.
pg_qualstats.track_constants:
Defaults to true. If true, each new value for each predicate will result in a new entry. Eg, WHERE id = 3 and WHERE id = 4 will results in two entries in pg_qualstats. If disabled, only one entry for WHERE id = ? will be kept. Turning this off drastically reduces the number of entries to keep, at the price of not getting any hindsight on most frequently used values.
pg_qualstats.sample_rate:
(Used to be “sample_ratio”) Defaults to -1, which means 1 / MAX_CONNECTIONS The ratio of queries that should be sampled. 1 means sample every single query, 0 basically deactivates the feature, and -1 is automatically sized to 1/ MAX_CONNECTIONS. For example, a sample_rate of 0.1 would mean one of out ten queries should be sampled.

SQL Objects

The extension defines the following objects: