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Displaying posts with tag: statement (reset)
Temporary Tables and Replication

I recently wrote about non-deterministic queries in the replication stream. That’s resolved by using either MIXED or ROW based replication rather than STATEMENT based.

Another thing that’s not fully handled by STATEMENT based replication is temporary tables. Imagine the following:

  1. Master: CREATE TEMPORARY TABLE rpltmpbreak (i INT);
  2. Wait for slave to replicate this statement, then stop and start mysqld (not just STOP/START SLAVE)
  3. Master: INSERT INTO rpltmpbreak VALUES (1);
  4. Slave: SHOW SLAVE STATUS \G

If for any reason a slave server shuts down and restarts after the temp table creation, replication will break because the temporary table will no longer exist on the restarted slave server. It’s obvious when you think about it, but nevertheless it’s quite …

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Non-Deterministic Query in Replication Stream

You might find a warning like the below in your error log:

130522 17:54:18 [Warning] Unsafe statement written to the binary log using statement format since BINLOG_FORMAT = STATEMENT. Statements writing to a table with an auto-increment column after selecting from another table are unsafe because the order in which rows are retrieved determines what (if any) rows will be written. This order cannot be predicted and may differ on master and the slave.
Statement: INSERT INTO tbl2 SELECT * FROM tbl1 WHERE col IN (417,523)

What do MariaDB and MySQL mean with this warning? The server can’t guarantee that this exact query, with STATEMENT based replication, will always yield identical results on the slave.

Does that mean that you have to use ROW based (or MIXED) replication? Possibly, but not necessarily.

For this type of query, it primarily refers to the fact that without …

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Temporary files, binlog_cache_size, and row-based binary logging

Even when the output of EXPLAIN doesn’t show “using temporary”, a temporary file may still be used in certain cases.

That’s not to say the query needs the temporary file to actually resolve the query (like what you’d see from the need for a derived table). But rather, the temporary file I’m speaking of is due to binary logging.

In particular, you can see this easily if using InnoDB, (most commonly) row-based binary logging, and you issue a large transaction, say a large UPDATE (large meaning something larger than the size of binlog_cache_size). In this case, you’ll notice a temporary file being …

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Using short if statement in programming

In many programing languages it is possible to shorten if statements using what’s called the ternary operator. It is sometimes referred as the “one line if statement” or the “short if statement”. This can help at times to produce cleaner code, however use this operator wisely as it is not always best to be used for more complicated statements.

PHP Example of an if statement

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if($nFoo > 0)
{
   echo "I'm at the work.";
}
else
{
   echo "I'm at home.";
}

PHP Example using the ternary operator

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echo $nFoo > 0 ? "I'm at the work." : "I'm at home.";

The expression (expr1) ? (expr2) : (expr3) evaluates to expr2 if expr1 evaluates to …

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Will you use row-based replication by default?



MySQL 5.1 introduces row based replication, a way of replicating data that fixes many inconsistencies of the statement based replication, the standard method used by MySQL so far.


The good: row based replication solves some problems when replicating the result of non deterministic functions, such as UUID() or NOW().
The bad: row-based replication may break existing applications, where you count on the quirks of statement based replication to execute conditionally (updates base on @@server_id, for example), and may perform badly on updates applied to very large tables.

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Showing entries 1 to 5