Performance on queries over DUAL table 2007-04-10 - By Jose Dillet
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Hello,
We have an application with JBoss and MaxDB 7.5.00.23 that I?m monitoring to
try to find performance bottlenecks. We?ve enabled the resource monitor as
explained in MaxDB performance tuning primer article. When I check the
results gathered, I see some strange results with queries over dual table:
SQL_STATEMENT CALL_COUNT ROWS_READ ROWS_QUAL PCT_ROWS
VIRTUAL_READS RUNTIME MIN_RUNTIME MAX_RUNTIME PHYSICAL_IO
SELECT * FROM DUAL 164 1128894 682042 0
3213934
347.271943 .000053 150.694322 58398
JBoss has a pool of connections, so, every time a connection is taken from
pool, jboss executes a SELECT * FROM DUAL to check that this connection is
still alive and working (this is configured at jboss datasource level). We
do this query because is the simplest query that could be done. But, as you
see, there was an execution with 150 secs, and cumulated runtime of 347
secs. I think this is too much time for so simple query. But, data of
ROWS_READ and ROWS_QUAL seems even stranger. Over a million rows read for
164 queries, and 600k rows qualified (rows that match criteria), but DUAL
has only 1 row (I?ve checked it and there isn?t other DUAL tables of other
users, so this query goes over DBA.DUAL, the one with only one row and one
field).
Do you think that this behaviour is a bug?
Best regards,
Jose Dillet
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