pgsql: Improve performance of index-only scans with many index columns.

Improve performance of index-only scans with many index columns.

StoreIndexTuple was a loop over index_getattr, which is O(N^2)
if the index columns are variable-width, and the performance
impact is already quite visible at ten columns. The obvious
move is to replace that with a call to index_deform_tuple ...
but that's *also* a loop over index_getattr. Improve it to
be essentially a clone of heap_deform_tuple.

(There are a few other places that loop over all index columns
with index_getattr, and perhaps should be changed likewise,
but most of them don't seem performance-critical. Anyway, the
rest would mostly only be interested in the index key columns,
which there aren't likely to be so many of. Wide index tuples
are a new thing with INCLUDE.)

Konstantin Knizhnik

Discussion: https://postgr.es/m/e06b2d27-04fc-5c0e-bb8c-ecd72aa24959@postgrespro.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/80b9e9c4664a020ebd14889046bd8d22a17d1ca6

Modified Files
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src/backend/access/common/indextuple.c | 69 ++++++++++++++++++++++++++++++--
src/backend/executor/nodeIndexonlyscan.c | 20 ++++-----
2 files changed, 72 insertions(+), 17 deletions(-)