Search Results for: where 1=1

Parallel Query for MySQL with Shard-Query

While Shard-Query can work over multiple nodes, this blog post focuses on using Shard-Query with a single node.  Shard-Query can add parallelism to queries which use partitioned tables.  Very large tables can often be partitioned fairly easily. Shard-Query can leverage partitioning to add paralellism, because each partition can be queried independently. Because MySQL 5.6 supports the […]

MySQL and Percona Server in LinkBench benchmark

Around month ago Facebook has announced the Linkbench benchmark that models the social graph OLTP workload. Sources, along with a very nice description of how to setup and run this benchmark, can be found here. We decided to run this benchmark for MySQL Server 5.5.30, 5.6.11 and Percona Server 5.5.30 and check how these servers […]

Recovering from a bad UPDATE statement

Did you just run an UPDATE against your 10 million row users table without a WHERE clause?  Did you know that in MySQL 5.5 that sometimes you can recover from a bad UPDATE statement?  This is possible if you are running in binlog_format=ROW ! Imagine this scenario:

We run an accidental UPDATE statement that […]

Timezone and pt-table-checksum

I recently worked through an issue with a client trying to detect data drift across some servers that were located in different timezones.  Unfortunately, several of the tables had timestamp fields and were set to a default value of CURRENT_TIMESTAMP.  From the manual, here is how MySQL handles timezone locality with timestamp fields: Values for TIMESTAMP columns are […]

EXPLAIN EXTENDED can tell you all kinds of interesting things

While many people are familiar with the MySQL EXPLAIN command, fewer people are familiar with “extended explain” which was added in MySQL 4.1 EXPLAIN EXTENDED can show you what the MySQL optimizer does to your query. You might not know this, but MySQL can dramatically change your query before it actually executes it. This process […]

Debugging problems with row based replication

MySQL 5.1 introduces row based binary logging. In fact, the default binary logging format in GA versions of MySQL 5.1 is ‘MIXED’ STATEMENT*;   The binlog_format  variable can still be changed per sessions which means it is possible that some of your binary log entries will be written in a row-based fashion instead of the […]

Indexes in MySQL

MySQL does not always make a right decision about indexes usage. Condsider a simple table:

; 250001 (V1)

; 83036 (V2) (execution time = 110 ms) That is index selectivity by condition (ID1=1) is V2/V1 = 0.3321 or 33.21% It is said (e.g. book “SQL Tuning”) if selectivity over 20% then a full table […]