pgsql: Avoid O(N^2) cost in ExecFindRowMark().

Avoid O(N^2) cost in ExecFindRowMark().

If there are many ExecRowMark structs, we spent O(N^2) time in
ExecFindRowMark during executor startup.  Once upon a time this was
not of great concern, but the addition of native partitioning has
squeezed out enough other costs that this can become the dominant
overhead in some use-cases for tables with many partitions.

To fix, simply replace that List data structure with an array.

This adds a little bit of cost to execCurrentOf(), but not much,
and anyway that code path is neither of large importance nor very
efficient now.  If we ever decide it is a bottleneck, constructing a
hash table for lookup-by-tableoid would likely be the thing to do.

Per complaint from Amit Langote, though this is different from
his fix proposal.

Discussion: https://postgr.es/m/468c85d9-540e-66a2-1dde-fec2b741e688@lab.ntt.co.jp

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f9eb7c14b08d2cc5eda62ffaf37a356c05e89b93

Modified Files
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src/backend/executor/execCurrent.c |  11 ++--
src/backend/executor/execMain.c    | 116 +++++++++++++++++++------------------
src/backend/executor/execUtils.c   |   9 ++-
src/include/nodes/execnodes.h      |  13 +++--
4 files changed, 82 insertions(+), 67 deletions(-)