Frederic Yhuel troubleshoots an issue:
We recently encountered a strange optimizer behaviour, reported by one of our customers:
Customer: “Hi Dalibo, we have a query that is very slow on the first execution after a batch process, and then very fast. We initially suspected a caching effect, but then we noticed that the execution plan was different.”
Dalibo: “That’s a common issue. Autoanalyze didn’t have the opportunity to process the table after the batch job had finished, and before the first execution of the query. You should run the
VACUUM ANALYZEcommand (or at leastANALYZE) immediately after your batch job.”Customer: “Yes, it actually solves the problem, but… your hypothesis is wrong. We looked at
pg_stat_user_tables, and are certain that the tables were not vacuumed or analyzed between the slow and fast executions. We don’t have a production problem, but we would like to understand.”Dalibo: “That’s very surprising! we would also like to understand…”
So let’s dive in!
Read on for a description of the issue and what Frederic and team found.