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C comments: improve description of GiST NSN and GistBuildLSN

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GiST indexes are complex, so adding more details in the code might help
someone.

Discussion: https://postgr.es/m/20210302164021.GA364@momjian.us
This commit is contained in:
Bruce Momjian
2021-03-10 17:03:10 -05:00
parent d87251048a
commit 845ac7f847
2 changed files with 12 additions and 4 deletions
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15
src/include/access/gist.h
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@ -51,13 +51,20 @@
#define F_HAS_GARBAGE (1 << 4) /* some tuples on the page are dead,
* but not deleted yet */
/* NSN - node sequence number, a special-purpose LSN */
/*
* NSN (node sequence number) is a special-purpose LSN which is stored on each
* index page in GISTPageOpaqueData and updated only during page splits. By
* recording the parent's LSN in GISTSearchItem.parentlsn, it is possible to
* detect concurrent child page splits by checking if parentlsn < child's NSN,
* and handle them properly. The child page's LSN is insufficient for this
* purpose since it is updated for every page change.
*/
typedef XLogRecPtr GistNSN;
/*
* A bogus LSN / NSN value used during index build. Must be smaller than any
* real or fake unlogged LSN, so that after an index build finishes, all the
* splits are considered completed.
* A fake LSN / NSN value used during index builds. Must be smaller than any
* real or fake (unlogged) LSN generated after the index build completes so
* that all splits are considered complete.
*/
#define GistBuildLSN ((XLogRecPtr) 1)