Skip to main content

UNION makes ALL the strength




Following the advice from a very expert consultant, I changed my views from an UNION between two select from the same table to UNION ALL and the performance improvements were simply amazing. Before doing so, I had verified that the query were returning different results with the INTERSECT command before doing so.

The difference is that UNION verifies that records are unique, while UNION ALL doesn't. The performance gain is very remarkable 

Comments

Post a Comment

Popular posts from this blog

Multiple controllers with Spring Boot

Remember, when you want to have multiple controllers with Spring Boot, you should always name them differently in the annotation, otherwise they will not work So these two together will NOT work (or just one of them will work) These two instead WILL work.
2 comments
Read more

WASService.sh and ulimit.... a tricky combo

From version 7 onward, the Application Server scripts and configuration scripts have the good manner to set by themselves the right ulimit valu. So ulimit is now less a problem than it was before. The problem is that, sometime when dealing with linux services,things do not always get right. For example you get exceptions such as [11/14/13 12:06:15:483 CET] 00000066 exception     E com.ibm.ws.wim.adapter.ldap.LdapAdapter authenticateWithPassword                                  com.ibm.websphere.wim.exception.WIMSystemException: CWWIM4520E  The 'javax.naming.CommunicationException: ldapus.ldap.domain.com:389 [Root exception is java.net.SocketException: Too many open files]' naming exception occurred during processing. (In case you are asking, yes, network connections are counted as open files) That's why I strongly suggest to modify your /etc/security/limits.conf (on RedHat) bef...
2 comments
Read more