Details
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Bug
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Status: Closed
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Major
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Resolution: Fixed
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1.8.0
Description
The computation of the hash value is basically the same whether for the Hash Table (used by Hash Agg, and Hash Join), or for distribution of rows at the exchange. As a result, a specific Hash Table (in a parallel minor fragment) gets only rows "filtered out" by the partition below ("upstream"), so the pattern of this filtering leads to a non uniform usage of the hash buckets in the table.
Here is a simplified example: An exchange partitions into TWO (minor fragments), each running a Hash Agg. So the partition sends rows of EVEN hash values to the first, and rows of ODD hash values to the second. Now the first recomputes the same hash value for its Hash table – and only the even buckets get used !! (Or with a partition into EIGHT – possibly only one eighth of the buckets would be used !! )
This would lead to longer hash chains and thus a poor performance !
A possible solution – add a distribution function distFunc (only for partitioning) that takes the hash value and "scrambles" it so that the entropy in all the bits effects the low bits of the output. This function should be applied (in HashPrelUtil) over the generated code that produces the hash value, like:
distFunc( hash32(field1, hash32(field2, hash32(field3, 0))) );
Tested with a huge hash aggregate (64 M rows) and a parallelism of 8 ( planner.width.max_per_node = 8 ); minor fragments 0 and 4 used only 1/8 of their buckets, the others used 1/4 of their buckets. Maybe the reason for this variance is that distribution is using "hash32AsDouble" and hash agg is using "hash32".
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