Binlogging and replication improvements¶
Due to continuous development, Percona Server incorporated a number of improvements related to replication and binary logs handling. This resulted in replication specifics, which distinguishes it from MySQL.
Temporary tables and mixed logging format¶
Summary of the fix:¶
As soon as some statement involving temporary table was met when using mixed binlog format, MySQL was switching to row-based logging of all statements the end of the session or until all temporary tables used in this session are dropped. It is inconvenient in case of long lasting connections, including replication-related ones. Percona Server fixes the situation by switching between statement-based and row-based logging as and when necessary.
Mixed binary logging format supported by Percona Server means that server runs in statement-based logging by default, but switches to row-based logging when replication would be unpredictable - in the case of a nondeterministic SQL statement that may cause data divergence if reproduced on a slave server. The switch is done upon any condition from the long list, and one of these conditions is the use of temporary tables.
Temporary tables are never logged using row-based format, but any statement, that touches a temporary table, is logged in row mode. This way all the side effects that temporary tables may produce on non-temporary ones are intercepted.
There is no need to use row logging format for any other statements solely because of the temp table presence. However MySQL was undertaking such an excessive precaution: once some statement with temporary table had appeared and the row-based logging was used, MySQL logged unconditionally all subsequent statements in row format.
Percona Server have implemented more accurate behavior: instead of switching to row-based logging until the last temporary table is closed, the usual rules of row vs statement format apply, and presence of currently opened temporary tables is no longer considered. This change was introduced with the fix of a bug #151 (upstream #72475).
Safety of statements with a
Summary of the fix:¶
MySQL considers all
UPDATE/DELETE/INSERT ... SELECT statements with
LIMIT clause to be unsafe, no matter wether they are really producing
non-deterministic result or not, and switches from statement-based logging
to row-based one. Percona Server is more accurate, it acknowledges such
instructions as safe when they include
ORDER BY PK or
condition. This fix has been ported from the upstream bug report