On Sun, Mar 06, 2022 at 12:37:00PM -0800, Zhihong Yu wrote: > The current design of pg_stat_statements doesn't have the concept of > observation. > > By observation I mean scenarios where pg_stat_statements is read by people > doing performance tuning. > > Here is one example (same query, q, is concerned). > At t1, q is performed, leaving one row in pg_stat_statements with mean_time > of 10. > At t2, operator examines pg_stat_statements and provides some suggestion > for tuning q (which is carried out). > At t3, q is run again leaving the row with mean_time of 9. > Now with two rows for q, how do we know whether the row written at t3 is > prior to or after implementing the suggestion made at t2 ?
Well, if pg_stat_statements is read by people doing performance tuning shouldn't they be able to distinguish which query text is the one they just rewrote? > Using other tools, a lot of the information in pg_stat_statements would be > duplicated to distinguish the counters recorded w.r.t. tuning operation. Yes, which is good. Your example was about rewriting a query, but what about other possibilities like creating an index, changing hash_mem_multiplier...? You won't get a new record and the mean_time will mostly be useless. If you take regular snapshot, then you will be able to compute the mean_time for each interval, and that will answer bot this scenario and the one in your example (since the 2nd row won't exist in the earlier snapshots).
