pgsql: Refactor ExecScan() to allow inlining of its core logic

Refactor ExecScan() to allow inlining of its core logic

This commit refactors ExecScan() by moving its tuple-fetching,
filtering, and projection logic into an inline-able function,
ExecScanExtended(), defined in src/include/executor/execScan.h.
ExecScanExtended() accepts parameters for EvalPlanQual state,
qualifiers (ExprState), and projection (ProjectionInfo).

Specialized variants of the execution function of a given Scan node
(for example, ExecSeqScan() for SeqScan) can then pass const-NULL for
unused parameters. This allows the compiler to inline the logic and
eliminate unnecessary branches or checks. Each variant function thus
contains only the necessary code, optimizing execution for scans
where these features are not needed.

The variant function to be used is determined in the ExecInit*()
function of the node and assigned to the ExecProcNode function pointer
in the node's PlanState, effectively turning runtime checks and
conditional branches on the NULLness of epqstate, qual, and projInfo
into static ones, provided the compiler successfully eliminates
unnecessary checks from the inlined code of ExecScanExtended().

Currently, only ExecSeqScan() is modified to take advantage of this
inline-ability. Other Scan nodes might benefit from such specialized
variant functions but that is left as future work.

Benchmarks performed by Junwang Zhao, David Rowley and myself show up
to a 5% reduction in execution time for queries that rely heavily on
Seq Scans. The most significant improvements were observed in
scenarios where EvalPlanQual, qualifiers, and projection were not
required, but other cases also benefit from reduced runtime overhead
due to the inlining and removal of unnecessary code paths.

The idea for this patch first came from Andres Freund in an off-list
discussion. The refactoring approach implemented here is based on a
proposal by David Rowley, significantly improving upon the patch I
(amitlan) initially proposed.

Suggested-by: Andres Freund <andres(at)anarazel(dot)de>
Co-authored-by: David Rowley <dgrowleyml(at)gmail(dot)com>
Reviewed-by: David Rowley <dgrowleyml(at)gmail(dot)com>
Reviewed-by: Junwang Zhao <zhjwpku(at)gmail(dot)com>
Tested-by: Junwang Zhao <zhjwpku(at)gmail(dot)com>
Tested-by: David Rowley <dgrowleyml(at)gmail(dot)com>
Discussion: https://postgr.es/m/CA+HiwqGaH-otvqW_ce-paL=96JvU4j+Xbuk+14esJNDwefdkOg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/fb9f955025f7609fd3da0d7e33b77438ddc765de

Modified Files
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src/backend/executor/execScan.c | 207 ++-----------------------------
src/backend/executor/nodeSeqscan.c | 115 ++++++++++++++++-
src/include/executor/execScan.h | 246 +++++++++++++++++++++++++++++++++++++
3 files changed, 365 insertions(+), 203 deletions(-)