DB2 - Problembeschreibung
Problem IT05904 | Status: Geschlossen |
INCORRECT RESULTS MIGHT BE PRODUCED IF HSJOIN HAS 2 NLJNs ON THE PROBE SIDE AND HSJN PROBE PUSH DOWN INTO THE SECOND NLJN DONE | |
Produkt: | |
DB2 FOR LUW / DB2FORLUW / A50 - DB2 | |
Problembeschreibung: | |
An SQL might produce incorrect results if all the following conditions are met: (a) The statement leverages a hash join (HSJOIN). (b) The plan has 2 nested loop joins (NLJOIN); one on the inner of another, and the other on the probe side of the HSJOIN. (c) An internal optimization on the probe side of the HSJN called a "push down" is implemented. The DB2 trace example below shows sqlrihsjnProbePD, which identifies this optimization taking place. We can use the EXPLAIN statement to see if the given SQL satisfies the conditions (a) and (b). The plan would have a graph like as below: HSJOIN / \ NLJOIN (x) / \ NLJOIN (y) Please note that this issue is likely to occur if NLJN(x) outer is not a real table, but a TABFNC: SYSIBM.GENROW Also, we can take db2trc to see if the HSJOIN is running into the condition (c), like as below: 1. Turn on the trace. db2trc on -f trc.dmp 2. Run the query. 3. Turn off the trace, then format it. db2trc off db2trc flw trc.dmp trc.flw 4. Search one sqlrihsjnProbePD that immediately returns with rc=1 inside sqlriFetch. grep -e sqlrihsjnProbePD -e sqlriFetch trc.flw 59255 | | | sqlriFetch entry 59417 | | | | | | | | | | | sqlrihsjnProbePD entry 59420 | | | | | | | | | | | sqlrihsjnProbePD exit [rc = 1] 50526 | | | sqlriFetch exit Note that the result might vary depending on the order and volume of the data. | |
Problem-Zusammenfassung: | |
**************************************************************** * USERS AFFECTED: * * All users * **************************************************************** * PROBLEM DESCRIPTION: * * See Error Description * **************************************************************** * RECOMMENDATION: * * Upgrade to DB2 version 10.5.0.5. * **************************************************************** | |
Local-Fix: | |
Users can disable all the push down by using the following setting. db2set DB2_TCG_DEFAULT_OPTIONS="set disable_pushdown on" Note that disabling push down can impact on query performance. | |
Lösung | |
The problem is first fixed in DB2 version 10.5.0.5. | |
Workaround | |
keiner bekannt / siehe Local-Fix | |
Weitere Daten | |
Datum - Problem gemeldet : Datum - Problem geschlossen : Datum - der letzten Änderung: | 03.12.2014 15.12.2014 09.04.2015 |
Problem behoben ab folgender Versionen (IBM BugInfos) | |
Problem behoben lt. FixList in der Version | |
10.5.0.5 |