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Change the implementation of hash join to attempt to avoid unnecessary

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work if either of the join relations are empty. The logic is:

(1) if the inner relation's startup cost is less than the outer
    relation's startup cost and this is not an outer join, read
    a single tuple from the inner relation via ExecHash()
      - if NULL, we're done

(2) read a single tuple from the outer relation
      - if NULL, we're done

(3) build the hash table on the inner relation
      - if hash table is empty and this is not an outer join,
        we're done

(4) otherwise, do hash join as usual

The implementation uses the new MultiExecProcNode API, per a
suggestion from Tom: invoking ExecHash() now produces the first
tuple from the Hash node's child node, whereas MultiExecHash()
builds the hash table.

I had to put in a bit of a kludge to get the row count returned
for EXPLAIN ANALYZE to be correct: since ExecHash() is invoked to
return a tuple, and then MultiExecHash() is invoked, we would
return one too many tuples to EXPLAIN ANALYZE. I hacked around
this by just manually detecting this situation and subtracting 1
from the EXPLAIN ANALYZE row count.
This commit is contained in:
Neil Conway
2005-06-15 07:27:44 +00:00
parent 4aaff55359
commit c119c5bd49
3 changed files with 169 additions and 62 deletions
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3
src/include/nodes/execnodes.h
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@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.133 2005/05/14 21:29:23 tgl Exp $
* $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.134 2005/06/15 07:27:44 neilc Exp $
*
*-------------------------------------------------------------------------
*/
@ -1218,6 +1218,7 @@ typedef struct HashState
HashJoinTable hashtable; /* hash table for the hashjoin */
List *hashkeys; /* list of ExprState nodes */
/* hashkeys is same as parent's hj_InnerHashKeys */
TupleTableSlot *firstTuple; /* tuple produced by ExecHash() */
} HashState;
/* ----------------