Peter, Matthias, thanks kindly for the good work on skipscans!
While admiring the recent performance improvements, I found a test case which fails after commit 21a152b37f36c9563d1b0b058fe1436baf578b4c. Please find a reproducible test case, attached.
Thanks!
On Fri, Apr 4, 2025 at 10:58 AM Peter Geoghegan <pg@bowt.ie> wrote:
Don't allow nbtree scans with skip arrays to end any primitive scan on its first leaf page without giving some consideration to how many times the scan's arrays advanced while changing at least one skip array (though continue not caring about the number of array advancements that only affected SAOP arrays, even during skip scans with SAOP arrays). Now when a scan performs more than 3 such array advancements in the course of reading a single leaf page, it is taken as a signal that the next page is unlikely to be skippable. We'll therefore continue the ongoing primitive index scan, at least until we can perform a recheck against the next page's finaltup.
Testing has shown that this new heuristic occasionally makes all the difference with skip scans that were expected to rely on the "passed first page" heuristic added by commit 9a2e2a28. Without it, there is a remaining risk that certain kinds of skip scans will never quite manage to clear the initial hurdle of performing a primitive scan that lasts beyond its first leaf page (or that such a skip scan will only clear that initial hurdle when it has already wasted noticeably-many cycles due to inefficient primitive scan scheduling).
