Mamta A. Satoor created DERBY-6156:
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Summary: Starting Derby 10.9, we do not not keep statistics for
single-column unique index. Check the code to see optimizer calculates correct
cost even with the missing stats for such indexes
Key: DERBY-6156
URL: https://issues.apache.org/jira/browse/DERBY-6156
Project: Derby
Issue Type: Task
Components: SQL, Store
Reporter: Mamta A. Satoor
With DERBY-3790(Investigate if request for update statistics can be skipped for
certain kind of indexes, one instance may be unique indexes based on one
column.) we have stopped keeping statistics for single column unique index. But
with DERBY-6045(in list multi-probe by primary key not chosen on tables with
>256 rows), we found that optimizer code was not adjusted to tackle the missing
stats and hence we were not picking index scan even though the single column
unique index was the right choice. DERBY-6045 has been fixed but we should make
sure there are no other places in the optimizer code where we have missed code
changes to account for missing statistics for single-column unique indexes.
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