Asking query performance questions

Question

What information should be provided when asking how to improve the performance of a query?

Answer 1

As many of the following as possible:

• Database product, precise version, and edition.
• Formatted text of the query/procedure/function.
• Current and desired performance levels.
• Things you can and cannot change (e.g. can add indexes, cannot change the query).
• Query plans
  • Ideally the post-execution variety, containing runtime information.
  • Estimated execution plans (if obtaining a post-execution plan is not practical).
  • Anonymized plans are acceptable, but may limit answer quality.
  • A picture of an execution is occasionally enough (but discouraged).
• A link to a demo that reproduces the core of the problem.
  • Or table, index, and constraint definitions (scripts).
• Hardware (e.g. memory/CPU/storage) and OS information.
• The tuning steps followed so far.
• The performance tag.

Further reading

• Slow Query Questions, Using EXPLAIN, Performance Optimization (PostgreSQL Wiki)
• How to create a Minimal, Complete, and Verifiable example (MCVE)
• Help us help you : keys to getting good answers.

Answer 2

Instructions for PostgreSQL Performance Questions

_{Served as description for the tag postgresql-performance before that was merged into the generic tag performance.}

_{Change was reverted and tag is back:postgresql-performance}

If you already know a way to get your output but need a faster solution then this tag may be appropriate. May concern techniques for a particular query or the setup of your hard- and software. Tag with postgresql as well.

Questions how to optimize simple queries should rather go to stackoverflow.com. See the DBA Help about suitable questions here.

Consider the basic advice on Performance Optimization and Slow Query Questions in the PostgreSQL Wiki before asking questions on this tag, including the "Things to try before you post" section. Using EXPLAIN is particularly important.

When posting questions, include:

• Your Postgres version, from SELECT version(), or at least your major version like 9.6 or 10.

• The full text of the query/queries. In a readable format and as brief as possible, but don't remove anything that might be relevant. Describe the expected result / include an example for simple cases.

• Table definition(s): Preferably a CREATE TABLE script showing data types and constraints for relevant columns and CREATE INDEX scripts for all relevant indexes. Or the output from \d+ tablename in psql.

• Cardinalities (rough number of rows) in involved tables. Rough distribution of critical values (if applicable).

• Query plan(s) obtained with EXPLAIN (BUFFERS, ANALYZE) or EXPLAIN ANALYZE in pg 9.0 or older. If possible paste them on explain.depesz.com and include links.

• If possible, add a link to an online demo at db<>fiddle, SQL Fiddle or similar, populated with the schema, some sample data, and the query.

• Only where relevant, a brief mention of your hardware and system, like:
  "CentOS 6.1, Xeon E5-2450 with 64GB RAM, 4-disk RAID 10 of Intel X-25E SSDs on Dell PERC H810 controller with flash write-back cache"
  Or more details if your question is about hardware.