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Clarify when to use PageSetLSN/PageGetLSN().

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Update README to explain prerequisites for
correct access to LSN fields of a page.
Independent chunk removed from checksums
patch to reduce size of patch.
This commit is contained in:
Simon Riggs 2012-12-03 11:59:25 +00:00
parent a068c391ab
commit 1c563a2ae1
1 changed files with 8 additions and 0 deletions
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8
src/backend/access/transam/README
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@ -564,6 +564,14 @@ something like
if (BufferIsValid(buffer0))
UnlockReleaseBuffer(buffer0);
Note that we must only use PageSetLSN/PageGetLSN() when we know the action
is serialised. Only Startup process may modify data blocks during recovery,
so Startup process may execute PageGetLSN() without fear of serialisation
problems. All other processes must only call PageSet/GetLSN when holding
either an exclusive buffer lock or a shared lock plus buffer header lock,
or be writing the data block directly rather than through shared buffers
while holding AccessExclusiveLock on the relation.
Due to all these constraints, complex changes (such as a multilevel index
insertion) normally need to be described by a series of atomic-action WAL
records. What do you do if the intermediate states are not self-consistent?