Performance on queries over DUAL table

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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