Depends on the contents of the database. The query optimizer doesn't know, at op... | Hacker News

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		Animats on Oct 13, 2014 \| parent \| context \| favorite \| on: Following a Select Statement Through Postgres Inte... Depends on the contents of the database. The query optimizer doesn't know, at optimization time, how many hits such a statement will produce. Given a database with only a few entries with the same name, the sort cost is tiny. If there are a lot of hits, it's expensive.

yangyang on Oct 13, 2014 [–]

It can estimate though, using per-column statistics. ANALYZE (and the autoanalyze background process) update these in PostgreSQL.

These stats are a huge part of cost-based query optimisation, which all major DBMSs do these days.

Details here: http://www.postgresql.org/docs/9.3/static/planner-stats.html

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