Explain Plan vs Actual Execution Plan

Hello,

Heres a question to ponder. While tuning a SQL statment for a user I noticed that the explain plan from SQL Analyze was not the same plan that was found when I used OEM Top Sessions (9.2.0.1) upon executing to collect execution stats. Database is on HP/UX 11 version Oracle 8.1.7

The stats were not "stale" yet. Monitoring is on for the tables in the query. The query would actually never return. I suspected that the stats were a bit off so I ran new ones and then SQL Analyze displayed a different explain plan and the plan reteived from top sessions while the SQL was running matched.

My question is is the Explain Plan and estimate or is the actual plan. I suspect that when an explain plan is created it uses statistics and the optimizer to determine the estimated plan and cost. However when the SQL is actually executed I suspect that a different plan may be generated as actual execution begins....or am I just wacked.

Either way the statistics when run created a proper plan that worked fine. But I wonder why the difference in plans... Received on Thu Mar 20 2003 - 14:22:27 CST