[GitHub] [incubator-superset] betodealmeida edited a comment on pull request #12130: feat: estimate query cost in Postgres

[GitBox <gi...@apache.org>]

betodealmeida edited a comment on pull request #12130:
URL: https://github.com/apache/incubator-superset/pull/12130#issuecomment-748339608


   > What's the unit of this cost? Seconds?
   
   > The costs are measured in arbitrary units determined by the planner's cost parameters (see Section 18.7.2). Traditional practice is to measure the costs in units of disk page fetches; that is, seq_page_cost is conventionally set to 1.0 and the other cost parameters are set relative to that. The examples in this section are run with the default cost parameters. [[reference](https://www.postgresql.org/docs/9.4/using-explain.html)]
   
   It's a relative value, so not super useful. But it's possible to collect stats about queries, and define a custom `QUERY_COST_FORMATTERS_BY_ENGINE` that maps from the relative values to something else. For example, you could run `EXPLAIN` every day in all recent queries, and use the results to built a histogram, so that the formatter says something like "this query is at the top 3% percentile of all queries".


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