How to Narrow Down Wrong Results Issues from Parallel Execution
(Doc ID 1340246.1)
Last updated on NOVEMBER 07, 2023
Applies to:
Oracle Database - Enterprise Edition - Version 10.2.0.1 and laterOracle Database Cloud Schema Service - Version N/A and later
Oracle Database Exadata Express Cloud Service - Version N/A and later
Gen 1 Exadata Cloud at Customer (Oracle Exadata Database Cloud Machine) - Version N/A and later
Oracle Cloud Infrastructure - Database Service - Version N/A and later
Information in this document applies to any platform.
Purpose
Introduction
This article covers wrong results problems that can be outpüt by parallel queries.
It provides action plans to narrow down the problem.
Please note that it is often useful to also search the Knowledge base with the term "wrong result" rather than "incorrect result" or "different result".
Background
There are a number of different factors that can cause an incorrect result, for example:
- there are problems with the execution plan
- parallel processes distribute the data incorrectly to the follow-up processes
Troubleshooting Steps
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In this Document
| Purpose |
| Troubleshooting Steps |
| 1. Problem with Slave Mapping Optimization |
| 2. Problems with Bloom Filter |
| a. Bloom Filters and Hash Joins |
| b. Bloom Filters and Partition Pruning |
| c. Bloom Filters for Predicate Pushdowns. |
| 3. Clumping of HASH JOIN ( only version before 10.2.0.5 ) |
| 4. Smart Scans ( only Exadata Systems ) |
| 5. Parallel Execution across multiple instances |
| 6. Execution plan that looks odd for parallel processing |
| Impact of workaround on performance |
| References |