12c Optimizer and mis-calculations.
Date: Wed, 11 May 2016 09:11:56 -0700
Message-ID: <009c01d1ab9f$db5d7560$92186020$_at_comcast.net>
Had to share: At one of my clients I was called in the middle of the night to look at a slow running query (9+ hours) on 12.1.0.2.0 on a X5 Exadata on a EDW.
When I pulled the execution plan from the cursor cache the calculated numbers were little off (look at rows/bytes/Cost) by about 9 orders of magnitude as the query actually only generates 2.9 million rows in about 6 minutes.
| Id | Operation | Name
| Rows | Bytes |TempSpc| Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib
|
| 0 | INSERT STATEMENT |
| | | | 29P(100)| | | |
|
| 1 | PX COORDINATOR |
| | | | | | | |
|
| 2 | PX SEND QC (RANDOM) | :TQ20001
| 2556T| 15E| | 3089M (5)| 20:26:25 | Q2,01 | P->S | QC (RAND)
|
This bad optimizer planning with estimated statistics or even 100% statistics has been troublesome of late on 12c especially for out of the box applications.
The variations in calculation of overall size normally appears in tempspc usage with and without adaptive planning in effect.
The extreme miscalculated values normally translate to queries that you kill otherwise they run for days before they die some unnatural death on their own.
The fix is normally just to reverse the table order of the joins the optimizer chose for the Fact versus the Dimension tables and then the query runs in minutes instead of days.
Matthew Parker
Chief Technologist
Dimensional DBA
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