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| -rw-r--r-- | src/backend/access/transam/README | 25 |
1 files changed, 15 insertions, 10 deletions
diff --git a/src/backend/access/transam/README b/src/backend/access/transam/README index 3a32471e951..f83526ccc36 100644 --- a/src/backend/access/transam/README +++ b/src/backend/access/transam/README @@ -575,16 +575,21 @@ 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? -The answer is that the WAL replay logic has to be able to fix things up. -In btree indexes, for example, a page split requires insertion of a new key in -the parent btree level, but for locking reasons this has to be reflected by -two separate WAL records. The replay code has to remember "unfinished" split -operations, and match them up to subsequent insertions in the parent level. -If no matching insert has been found by the time the WAL replay ends, the -replay code has to do the insertion on its own to restore the index to -consistency. Such insertions occur after WAL is operational, so they can -and should write WAL records for the additional generated actions. +records. The intermediate states must be self-consistent, so that if the +replay is interrupted between any two actions, the system is fully +functional. In btree indexes, for example, a page split requires a new page +to be allocated, and an insertion of a new key in the parent btree level, +but for locking reasons this has to be reflected by two separate WAL +records. Replaying the first record, to allocate the new page and move +tuples to it, sets a flag on the page to indicate that the key has not been +inserted to the parent yet. Replaying the second record clears the flag. +This intermediate state is never seen by other backends during normal +operation, because the lock on the child page is held across the two +actions, but will be seen if the operation is interrupted before writing +the second WAL record. The search algorithm works with the intermediate +state as normal, but if an insertion encounters a page with the +incomplete-split flag set, it will finish the interrupted split by +inserting the key to the parent, before proceeding. Writing Hints ------------- |