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Measure the number of all-visible pages for use in index-only scan costing.
Add a column pg_class.relallvisible to remember the number of pages that were all-visible according to the visibility map as of the last VACUUM (or ANALYZE, or some other operations that update pg_class.relpages). Use relallvisible/relpages, instead of an arbitrary constant, to estimate how many heap page fetches can be avoided during an index-only scan. This is pretty primitive and will no doubt see refinements once we've acquired more field experience with the index-only scan mechanism, but it's way better than using a constant. Note: I had to adjust an underspecified query in the window.sql regression test, because it was changing answers when the plan changed to use an index-only scan. Some of the adjacent tests perhaps should be adjusted as well, but I didn't do that here.
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@ -1743,6 +1743,7 @@ _outRelOptInfo(StringInfo str, RelOptInfo *node)
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WRITE_NODE_FIELD(indexlist);
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WRITE_UINT_FIELD(pages);
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WRITE_FLOAT_FIELD(tuples, "%.0f");
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WRITE_FLOAT_FIELD(allvisfrac, "%.6f");
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WRITE_NODE_FIELD(subplan);
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WRITE_NODE_FIELD(subroot);
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WRITE_NODE_FIELD(baserestrictinfo);
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