transitive closure

The Cost Based Optimizer (CBO) supports since at least Oracle 9i the automatic generation of additional predicates based on transitive closure.

In principle this means:

If a = b and b = c then the CBO can infer a = c

As so often with these optimizations the purpose of these automatically generated additional predicates is to allow the optimizer finding potentially more efficient access paths, like an index usage or earlier filtering reducing the amount of data to process.

If you think 16MB tables are so small that can’t be root cause of significant performance problems with SELECT queries, then read about my today’s headache – a story of 16MB table worth something like 270 seconds of response time. It began mostly as usual: an application web page didn’t show up within 5 minutes. [...]