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IDS Forum

Re: UPDATE STATISTICS low in parallel?

Posted By: JACQUES RENAUT
Date: Monday, 10 April 2017, at 3:14 p.m.

In Response To: UPDATE STATISTICS low in parallel? (BENJAMIN THOMPSON)

Original post:

Hi,

This has been bothering me for a while but has until now been a low priority. I am using a special build of 11.70.FC7W1 but the feature has been there since at least 11.50.

https://www.ibm.com/support/knowledgecenter/en/SSGU8G_11.70.0/com.ibm.perf.doc/ids_prf_638.htm

According to the above section of the manual UPDATE STATISTICS LOW can work in parallel when:

- PDQPRIORITY is set.
- Indices are fragmented.

The documented talks about an "extremely large" database: I am not sure if this is a requirement or is just a suggested situation in which it might be useful. I presume the latter. Anyway the database as a whole I am working with is in excess of 10 Tb.

The document is also vague about whether it is the actual PDQ_PRIORITY that matters or the effective one, i.e. what is asked for reduced by MAX_PDQPRIORITY.

Try as I might, I cannot see any evidence of parallelism when the above conditions are met. While my session running UPDATE STATISTICS LOW has a non-zero PDQ value, it has one thread, there are no threads running I can't account for and the memory grant manager shows no PDQ queries.

The table I am testing with has 1,253,635,127 rows, uses 7691017 8 kb pages and is partitioned by round-robin 8 ways. It has three indices on a single column each:

- serial column, fragmented 8 ways by mod(value).
- int column, fragmented 8 ways by mod(value).
- datetime column, fragmented 12 ways by month(value).

I feel this meets the criteria. I have also tried with and without USTLOW_SAMPLE set.

Is it just a bug or documentation bug?

Ben.

Response:

What are you actually setting PDQPRIORITY to, or what values have you tried? I was looking at the code and I also think it isn't affected by MAX_PDQPRIORITY, but I will admit I might have missed that (I didn't look super close at if anything might have affected the value of where the code is looking for the value). But the code that decides how many threads to start up is affected by what value you have set for PDQPRIORITY and the number of fragments the index is split over. It looks like you have 8 fragments for your index. To get more then 1 thread to scan the index fragments in this case, I think you would need to have PDQPRIORITY set to at least 3. I guess I could assume that you tried setting PDQPRIORITY to 10 like the link you provided, but I'd like confirmation. The only other thing I can think of is, it's somehow not getting into the piece of code that tries to do the update stats in parallel, but the only things that should be preventing that is that if the index isn't fragmented, it's not an index on a user defined type and some memory structures can get allocated and set up.

Jacques Renaut
IBM Informix Adavanced Support

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