Using QueryID for Joins on DBQL Tables
Because each DBQL table has its own separate cache, the system may fill up the cache for one table and log to that DBQL table before it logs into another DBQL table. This results in slightly different CollectTimeStamp values. Therefore, do not include CollectTimeStamp fields in joins.
If you need to join different DBQL tables, use the QueryID field. If you plan on comparing values between different DBQL tables, you can flush the DBQL caches to the dictionary tables. See “Options for Flushing the DBQL Cache” on page 372.
Note: Although it is rare, you may see orphan rows in join results involving the DBQLObjTbl, DBQLExplainTbl, or DBQLXMLTbl tables. An orphan row is one that does not appear in the default log table (DBQLogTbl), but has an entry in one of the three other tables.
Teradata recommends that you ignore the orphan rows and do not copy them over when you copy the DBQL tables out of DBC and into temporary or history tables in another database.