Subject: pgsql: Fix some performance issues in GIN query startup.

From: Tom Lane <[email-masked]>
Date: Thu, 06 Mar 2025 16:54:48 +0000

Fix some performance issues in GIN query startup.

If a GIN index search had a lot of search keys (for example,
"jsonbcol ?| array[]" with tens of thousands of array elements),
both ginFillScanKey() and startScanKey() took O(N^2) time.
Worse, those loops were uncancelable for lack of CHECK_FOR_INTERRUPTS.

The problem in ginFillScanKey() is the brute-force search key
de-duplication done in ginFillScanEntry().  The most expedient
solution seems to be to just stop trying to de-duplicate once
there are "too many" search keys.  We could imagine working harder,
say by using a sort-and-unique algorithm instead of brute force
compare-all-the-keys.  But it seems unlikely to be worth the trouble.
There is no correctness issue here, since the code already allowed
duplicate keys if any extra_data is present.

The problem in startScanKey() is the loop that attempts to identify
the first non-required search key.  In the submitted test case, that
vainly tests all the key positions, and each iteration takes O(N)
time.  One part of that is that it's reinitializing the entryRes[]
array from scratch each time, which is entirely unnecessary given
that the triConsistentFn isn't supposed to scribble on its input.
We can easily adjust the array contents incrementally instead.
The other part of it is that the triConsistentFn may itself take
O(N) time (and does in this test case).  This is all extremely
brute force: in simple cases with AND or OR semantics, we could
know without any looping whatever that all or none of the keys
are required.  But GIN opclasses don't have any API for exposing
that knowledge, so at least in the short run there is little to
be done about that.  Put in a CHECK_FOR_INTERRUPTS so that at
least the loop is cancelable.

These two changes together resolve the primary complaint that
the test query doesn't respond promptly to cancel interrupts.
Also, while they don't completely eliminate the O(N^2) behavior,
they do provide quite a nice speedup for mid-sized examples.

Bug: #18831
Reported-by: Niek <[email-masked]>
Author: Tom Lane <[email-masked]>
Discussion: https://postgr.es/m/[email-masked]
Backpatch-through: 13

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/308d0d443770a0dfe49e7e253792dc6b3a149fb0

Modified Files
--------------
src/backend/access/gin/ginget.c  | 10 ++++++----
src/backend/access/gin/ginscan.c |  7 ++++++-
2 files changed, 12 insertions(+), 5 deletions(-)