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nodeMergejoin.c
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Tom Lane authored
is treated like end-of-input, if nulls sort last in that column and we are not doing outer-join filling for that input. In such a case, the tuple cannot join to anything from the other input (because we assume mergejoinable operators are strict), and neither can any tuple following it in the sort order. If we're not interested in doing outer-join filling we can just pretend the tuple and its successors aren't there at all. This can save a great deal of time in situations where there are many nulls in the join column, as in a recent example from Scott Marlowe. Also, since the planner tends to not count nulls in its mergejoin scan selectivity estimates, this is an important fix to make the runtime behavior more like the estimate. I regard this as an omission in the patch I wrote years ago to teach mergejoin that tuples containing nulls aren't joinable, so I'm back-patching it. But only to 8.3 --- in older versions, we didn't have a solid notion of whether nulls sort high or low, so attempting to apply this optimization could break things.
Tom Lane authoredis treated like end-of-input, if nulls sort last in that column and we are not doing outer-join filling for that input. In such a case, the tuple cannot join to anything from the other input (because we assume mergejoinable operators are strict), and neither can any tuple following it in the sort order. If we're not interested in doing outer-join filling we can just pretend the tuple and its successors aren't there at all. This can save a great deal of time in situations where there are many nulls in the join column, as in a recent example from Scott Marlowe. Also, since the planner tends to not count nulls in its mergejoin scan selectivity estimates, this is an important fix to make the runtime behavior more like the estimate. I regard this as an omission in the patch I wrote years ago to teach mergejoin that tuples containing nulls aren't joinable, so I'm back-patching it. But only to 8.3 --- in older versions, we didn't have a solid notion of whether nulls sort high or low, so attempting to apply this optimization could break things.
