On 22/01/14 03:53, Tom Lane wrote:
Jon Nelson <jnelson+pg...@jamponi.net> writes:
A rough summary of the patch follows:
- a GUC variable enables or disables this capability
- in nodeAgg.c, eliding duplicate tuples is enabled if the number of
distinct columns is equal to the number of sort columns (and both are
greater than zero).
- in createplan.c, eliding duplicate tuples is enabled if we are
creating a unique plan which involves sorting first
- ditto planner.c
- all of the remaining changes are in tuplesort.c, which consist of:
+ a new macro, DISCARDTUP and a new structure member, discardtup, are
both defined and operate similar to COMPARETUP, COPYTUP, etc...
+ in puttuple_common, when state is TSS_BUILDRUNS, we *may* simply
throw out the new tuple if it compares as identical to the tuple at
the top of the heap. Since we're already performing this comparison,
this is essentially free.
+ in mergeonerun, we may discard a tuple if it compares as identical
to the *last written tuple*. This is a comparison that did not take
place before, so it's not free, but it saves a write I/O.
+ We perform the same logic in dumptuples
[ raised eyebrow ... ] And what happens if the planner drops the
unique step and then the sort doesn't actually go to disk?
I don't think Jon was suggesting that the planner drop the unique step.
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