Why aren't hashes preserved by joins?

Roberto Cornacchia roberto.cornacchia at gmail.com
Thu Apr 21 18:07:48 CEST 2016


I just noticed that PERSISTENTHASH (checked by BAThash) in gdk_private is
not defined (commented). But this wouldn't explain the different between
selection and join.



On 21 April 2016 at 18:02, Roberto Cornacchia <roberto.cornacchia at gmail.com>
wrote:

> 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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