Derby
  1. Derby
  2. DERBY-1905

Optimizer cost estimates for subqueries are way (way) too high.

    Details

    • Type: Bug Bug
    • Status: Open
    • Priority: Major Major
    • Resolution: Unresolved
    • Affects Version/s: 10.1.3.2, 10.2.1.6, 10.3.1.4
    • Fix Version/s: None
    • Component/s: SQL
    • Urgency:
      Normal
    • Issue & fix info:
      High Value Fix, Repro attached
    • Bug behavior facts:
      Performance

      Description

      If I run the attached repro (pulled from DERBY-1866) with derby.optimizer.noTimeout=true (meaning the optimizer should take all the time it needs to try out all possible plans within the restrictions of the optimizer overrides), the estimated cost shown in derby.log (if I log the query plan) is over 600k and the estimated row count is over 520k.

      If, as the code in OptimizerImpl seems to expect, the unit for a cost estimate is milliseconds, then the optimize here is guessing that the query will take over 10 minutes to execute and will return a half-million rows. But in truth the combined time for compilation AND execution is just a couple of seconds, and only 15 rows are returned.

      That suggests to me a rather serious problem with the optimizer cost estimates for subqueries.

      Among other things this can have a major impact on the optimizer's timeout mechanism for very deeply-nested queries. The optimizer will continue to try out different combinations of indexes/joinOrders/joinStrategies until it either exhausts them all or until the number of milliseconds spent optimizing is greater than the "best cost" estimate so far. In the case of the repro for this issue, the optimizer quickly exhausts all of the options and so it finishes in a fair amount of time.

      But in larger queries where there are far more combinations to try (see esp. queries attached to DERBY-1205, DERBY-1777), these severly inflated cost estimates get very large very quickly (sometimes even reaching infinity--see DERBY-1259, DERBY-1260) and so the optimizer just keeps on optimizing and never times out. The result is that for queries like those in DERBY-1777, the optimizer can spend literally hours optimizing a query which then executes in a matter of seconds.

      I'm still investigating this, but preliminary examination suggests that part of the problem is in some way related to the treatment of "outer costs" by the optimizer--and in particular, it looks like the optimizer is perhaps too aggressive in multiplying cost estimates by row counts pulled from "outer costs". That's just my first guess at the problem, though; there could of course be far more to it than that...

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                Updated:

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