pgsql: Generate index-only scan tuple descriptor from the plan node's i

Generate index-only scan tuple descriptor from the plan node's indextlist.

Dept. of second thoughts: as long as we've got that tlist hanging around
anyway, we can apply ExecTypeFromTL to it to get a suitable descriptor for
the ScanTupleSlot. This is a nicer solution than the previous one because
it eliminates some hard-wired knowledge about btree name_ops, and because
it avoids the somewhat shaky assumption that we needn't set up the scan
tuple descriptor in EXPLAIN_ONLY mode. It doesn't change what actually
happens at run-time though, and I'm still a bit nervous about that.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/cb6771fb32cbdb11c8d84b7d62ee940bdba38d52

Modified Files
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src/backend/executor/nodeIndexonlyscan.c | 68 +++++++-----------------------
1 files changed, 16 insertions(+), 52 deletions(-)