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What is an EXPLAIN in DB2.
How and what is the steps followed in EXPLAIN command. Can
all the queries be used in EXPLAIN command to check the
performance.
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Answer Posted / ryan
Whenever we run a query,DB2 creates an access plan that specifies how it will access the requested data. This is created whenever the sql is compiled at bind time for static sql and before execution for dynamic sql.
DB2 bases the access paths on the SQL statements and also on the statistics and configuration parameters of the system.
Even when an sql is made efficient, it can become inefficient as data grows. So, we need to run DB2 runstats so as to keep updated statistics . DB2 config and storage can change and plans can be rebound. Db2 explain gives us the info for the plan, package, or SQL statement when it is bound. The output of the EXPLAIN is stored in user-created table called plan table. Whenever we want to tune a query, we need to go and check the plan table so as to get an idea of the access path DB2 optimizer is using.
http://mframes.blogspot.in/2015/01/db2-explain-and-plan-table-column-names.html
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