DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

Full SQL statement text trace record
DB2 V8 provides a new IFCID 350, which is both similar and in addition to IFCID 63, which
traces the entire SQL statement. Previous versions of DB2 limited the SQL statement text to a
maximum of 5,000 bytes (the maximum length of a trace field).
Since there is a 5,000 byte limit on a single data item being traced, a “repeating group" of
5,000 byte items are placed in each IFCID 350 record up to a maximum of 6. If that is not
enough, multiple IFCID 350 records are produced. IFCID 350 is written under the same
circumstances that trigger the writing of IFCID 63. PREPARE attributes are also traced.
Enhanced trace recording for PREPARE attribute
Since DB2 V7, you can change the attributes of a cursor or SELECT statement on the
PREPARE statement. To do so, you use the ATTRIBITES clause on the PREPARE. For
example, it allows you to change a cursor’s isolation level by providing a new attribute string.
However, in V7 the attribute string is not always present in IFCID records, which makes
problem determination and performance analysis more complex.
DB2 V8 adds the attribute string to the following trace records:
► IFCID 63: (which contains the first 5,000 bytes of the SQL statement text) now also
contains the attribute string on a short prepare.
► IFCID 124: (which is used to obtain information about the currently executing SQL
statement through a READS request) now also contains the attributes string.
► IFCID 317: (which is used to retrieve the SQL statement string of a statement in the
dynamic statement cache) now also contains the attributes string.
Add cached statement ID to IFCID 124
IFCID 124 provides information about the currently executing SQL statement through the
READS interface. DB2 V8 enhances IFCID 124 to also include the dynamic cached SQL
statement ID. The 4 byte field QW0124ST was added to contain the cached dynamic SQL
statement identifier.
This enhancement works together with another enhancement in DB2 V8 that allows you to
Explain SQL statements from the dynamic statement cache. You can retrieve the information
about the currently executing SQL statement via IFCID 124. That now contains the SQL
statement ID of the statement in the dynamic statement cache, and then explains the SQL
statement from the dynamic statement cache using that statement ID.
For more information on how to explain a statement from the dynamic statement cache, see
section 4.20, “EXPLAIN STMTCACHE” in the book DB2 UDB for z/OS Version 8: Everything
You Ever Wanted to Know, ... and More, SG24-6079.
Agent level workfile tracing
DB2 V8 introduces a new IFCID 342 to record workfile and TEMP database usage at an
application level. This allows you to take corrective action, such as adding space or cancelling
a runaway query that is using a lot of space in the databases and preventing other queries
from running successfully even they only need very little space.
When active, IFCID 342 is written whenever space is allocated or deallocated in the workfile
or TEMP database. It gives information about the total amount of space being used by the
agent currently and the maximum space that the agent may have used at any given point in
time in the workfile or TEMP database. The trace also records the total amount of space
being used for indexes on Declared Temporary Tables.
Chapter 4. DB2 subsystem performance 211