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pg_sortstats

/!\ This extension is under development and not production ready. Use at your own risk.

PostgreSQL extension to gather and cumulate various statistics about sorts, and estimate how much work_mem would be needed to have the sort done in memory.

Statistics are aggregated per queryid (query identifier as computed by pg_stat_statements), userid, dbid and sort_key (the textual representation of the sort being performed).

pg_stat_statements is needed to provide the queryid field.

Installation

Compiling

The module can be built using the standard PGXS infrastructure. For this to work, the pg_config program must be available in your $PATH. Instruction to install follows::

git clone https://github.com/powa-team/pg_sortstats.git cd pg_sortstats make make install

NOTE: The "make install" part may require root privilege.

PostgreSQL setup

The extension is now available. But, as it requires some shared memory to hold its counters, the module must be loaded at PostgreSQL startup. Thus, you must add the module to shared_preload_libraries in your postgresql.conf. You need a server restart to take the change into account. As this extension depends on pg_stat_statements, it also need to be added to shared_preload_libraries.

Add the following parameters into you postgresql.conf::

postgresql.conf

shared_preload_libraries = 'pg_stat_statements,pg_sortstats'

Once your PostgreSQL cluster is restarted, you can install the extension in every database where you need to access the statistics::

mydb=# CREATE EXTENSION pg_sortstats;

Usage

The pg_sortstats view provides the following fields:

fieldname description
queryid pg_stat_statements' queryid
userid user identifier
dbid database identifier
sort_key the textual sort expression
lines total number of lines the sort node had in input
lines_to_sort total number of lines the sort node has to sort (different when there's a LIMIT)
work_mems total size of needed work_mem that was estimated to perform the sort in memory
topn_sorts total number of sorts done using Top-N heapsort algorithm
quicksorts total number of sorts done using quicksort algorithm
external_sorts total number of sorts done using external sort algorithm
external_merges total number of sorts done using external merge algorithm
nbtapes total number of tapes used for external merge sorts
space_disk total disk space used to perform the sort
space_memory total memory space used to perform the sort
non_parallels total number of sorts not done in parallel
nb_workers total number of processes used to perform the sort

The pg_sortstats(showtext) can be used instead, passing false as paremeter if you don't need the sort_key field.

The pg_sortstats_reset() function can be used to remove all stored statistics.