You can monitor queries using the Teradata Viewpoint Query Monitor portlet and other tools and utilities to identify poorly performing queries. Poor query performance is usually due to:
|Stale statistics||The optimizer uses statistics to plan query execution. If the statistics are out of date, the optimizer may create a poor plan.
Recommendation: Review tables to see whether statistics are out of date, and recollect statistics if required, as shown in Improving Query Performance Using COLLECT STATISTICS: Application DBAs.
|Badly skewed data||Causes table rows to be distributed unevenly among AMPs and queries to execute unevenly, which slows processing time.
Recommendation: Monitor table skew. Redefine the primary index if required. See Finding and Fixing Skewed Tables.
|Poor query construction||Poor SQL syntax or missed punctuation can result in bad queries.
|Concurrency issues||Some queries are delayed or prevented from executing because of lock contentions, or the relative priorities of concurrent queries.