Why aren't hashes preserved by joins?

Roberto Cornacchia roberto.cornacchia at gmail.com
Thu Apr 21 18:02:55 CEST 2016


I noticed a difference between
- a hash-based string selection from a persistent, read-only table
- a hash-based join on the same table and same column

They both build a hash on the same string column (verified with gdb), but
the select can reuse the hash (second call is almost free), while the join
keeps rebuilding the hash.

Is this expected?

Roberto
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