Answer / yuvaevergreen

>>Explain can be used to improve the performance of queries.
>>Explain command populates three tables
PLAN_TABLE,DSN_STATEMNT_TABLE,DSN_FUNCTION_TABLE. These
tables contains various info about the queries which can be
used for improving the performance.
>>Step1:PLAN_TABLE must be created.
>>Step2:Use explain sql or explain option in Bind to
populate the tables.
>>For example,explain plan set queryno=01 for select empname
from employee;
This would populate plan and statement table.Assigned
queryno would be 01.
>>Alternately EXPLAIN(YES) in BIND option.
>>Only select,insert,update,select for update,delete, delete
current of cursor,update current of cursor can be used in
explain.

Answer / s.v.venkata sivaprasad

explain is the db2 tool which used to improve db2
performance and provides best access path to package

Answer / 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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