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Method R Discussion Group

How do we read the eplain/execution plan? From top to bottom OR from bottom to top?

It’s a little bit of both top-down and inside-out-bottom-to-top.  The steps are created top down, however a step can’t complete until all of it’s dependent (or child) steps finish first.

Take this plan:
0 | SELECT STATEMENT
1 |  HASH JOIN
2 |    HASH JOIN
3 |        TABLE ACCESS FULL
4 |        TABLE ACCESS FULL
5 |    TABLE ACCESS FULL

The first step to start is step 1, but it can’t complete until it gets some input on what to hash join together.  When steps are at the same level then they complete in ID order.  So the inner most level is steps 3 and 4.  Step 3 will finish first then step 4.  Now the Hash join on step 2 completes then step 5. Now everything that is need for step 1 to complete is done so step one completes. 

I hope this helps.