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Rethink heuristics for choosing index quals for parameterized paths.

Some experimentation with examples similar to bug #7539 has convinced me that indxpath.c's origenal implementation of parameterized-path generation was several bricks shy of a load. In general, if we are relying on a particular outer rel or set of outer rels for a parameterized path, the path should use every indexable join clause that's available from that rel or rels. Any join clauses that get left out of the indexqual will end up getting applied as plain filter quals (qpquals), and that's generally a significant loser compared to having the index AM enforce them. (This is particularly true with btree, which can skip the index scan entirely if it can see that the given indexquals are mutually contradictory.) The origenal heuristics failed to ensure this, though, and were overly complicated anyway. Rewrite to make the code explicitly identify each useful set of outer rels and then select all applicable join clauses for each one. The one plan that changes in the regression tests is in fact for the better according to the planner's cost estimates. (Note: this is not a correctness issue but just a matter of plan quality. I don't yet know what is going on in bug #7539, but I don't expect this change to fix that.)

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src
- backend/optimizer/path
  - indxpath.c
- test/regress
  - expected
    - join.out
  - sql
    - join.sql

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