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Fix GiST README's explanation of the NSN cross-check.

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The text got the condition backwards, it's "NSN > LSN", not "NSN < LSN".
While we're at it, expand it a little for clarity.

Reviewed-by: Daniel Gustafsson
Discussion: https://www.postgresql.org/message-id/4cb46e18-e688-524a-0f73-b1f03ed5d6ee@iki.fi
This commit is contained in:
Heikki Linnakangas 2023-09-19 11:53:51 +03:00
parent 35e8ba3e54
commit 459439f728
1 changed files with 4 additions and 4 deletions
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src/backend/access/gist/README
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@ -269,10 +269,10 @@ should be visited too. When split inserts the downlink to the parent, it
clears the F_FOLLOW_RIGHT flag in the child, and sets the NSN field in the
child page header to match the LSN of the insertion on the parent. If the
F_FOLLOW_RIGHT flag is not set, a scan compares the NSN on the child and the
LSN it saw in the parent. If NSN < LSN, the scan looked at the parent page
before the downlink was inserted, so it should follow the rightlink. Otherwise
the scan saw the downlink in the parent page, and will/did follow that as
usual.
LSN it saw in the parent. If the child's NSN is greater than the LSN seen on
the parent, the scan looked at the parent page before the downlink was
inserted, so it should follow the rightlink. Otherwise the scan saw the
downlink in the parent page, and will/did follow that as usual.
A scan can't normally see a page with the F_FOLLOW_RIGHT flag set, because
a page split keeps the child pages locked until the downlink has been inserted