pgsql: Minor refactoring in heap_page_prune

Minor refactoring in heap_page_prune

Pass 'page', 'blockno' and 'maxoff' to heap_prune_chain() as
arguments, so that it doesn't need to fetch them from the buffer. This
saves a few cycles per chain.

Remove the "if (off_loc != NULL)" checks, and require the caller to
pass a non-NULL 'off_loc'. Pass a pointer to a dummy local variable
when it's not needed. Those checks are cheap, but it's still better to
avoid them in the per-chain loops when we can do so easily.

The CPU time saving from these changes are hardly measurable, but
fewer instructions is good anyway, so why not. I spotted the potential
for these while reviewing Melanie Plageman's patch set to combine
prune and freeze records.

Discussion: https://www.postgresql.org/message-id/CAAKRu_abm2tHhrc0QSQa%3D%3DsHe%3DVA1%3Doz1dJMQYUOKuHmu%2B9Xrg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9917e79d995c845adfe9b6cb3ea5cb7af7cec654

Modified Files
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src/backend/access/heap/pruneheap.c | 37 +++++++++++++++----------------------
1 file changed, 15 insertions(+), 22 deletions(-)