database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-06-23 14:01:44 +03:00
Code Activity

Update README, we don't do post-recovery cleanup actions anymore.

Browse Source 
transam/README explained how B-tree incomplete splits were tracked and
fixed after recovery, as an example of handling complex actions that need
multiple WAL records, but that's not how it works anymore. Explain the new
paradigm.
This commit is contained in:
Heikki Linnakangas
2014-05-17 13:48:52 +03:00
parent 7894ac5004
commit a3655dd4a5
1 changed files with 15 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files

25
src/backend/access/transam/README
View File

@ -575,16 +575,21 @@ while holding AccessExclusiveLock on the relation.
Due to all these constraints, complex changes (such as a multilevel index 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 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? records. The intermediate states must be self-consistent, so that if the
The answer is that the WAL replay logic has to be able to fix things up. replay is interrupted between any two actions, the system is fully
In btree indexes, for example, a page split requires insertion of a new key in functional. In btree indexes, for example, a page split requires a new page
the parent btree level, but for locking reasons this has to be reflected by to be allocated, and an insertion of a new key in the parent btree level,
two separate WAL records. The replay code has to remember "unfinished" split but for locking reasons this has to be reflected by two separate WAL
operations, and match them up to subsequent insertions in the parent level. records. Replaying the first record, to allocate the new page and move
If no matching insert has been found by the time the WAL replay ends, the tuples to it, sets a flag on the page to indicate that the key has not been
replay code has to do the insertion on its own to restore the index to inserted to the parent yet. Replaying the second record clears the flag.
consistency. Such insertions occur after WAL is operational, so they can This intermediate state is never seen by other backends during normal
and should write WAL records for the additional generated actions. 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 Writing Hints
------------- -------------