Here’s a live one from OTN – here are a couple of extracts from the problem statement:
We’re experiencing an issue where it seems that the query plan changes from day to day for a particular procedure that runs once a night.
It’s resulting in a performance variance of 10 second completion time vs 20 minutes (nothing in between).
It started occurring about 2 months ago and now it’s becoming more prevalent where the bad query plan is coming up more often.
I noticed that the query plans vary for a simple query.
We do run gather statistics every night. (DBMS_STATS.GATHER_SCHEMA_STATS (ownname=>sys_context( ‘userenv’, ‘current_schema’ ), estimate_percent => 1);)
The query and two execution plans look like this:
I’ve just responded to the call for items for the “IOUG Quick Tips” booklet for 2015 – so it’s probably about time to post the quick tip that I put into the 2014 issue. It’s probably nothing new to most readers of the blog, but sometimes an old thing presented in a new way offers fresh insights or better comprehension.
A histogram, created in the right way, at the right time, and supported by the correct client-side code, can be a huge benefit to the optimizer; but if you don’t create and use them wisely they can easily become a source of inconsistent performance, and the automatic statistics gathering can introduce an undesirable overhead during the overnight batch. This note explains how you can create histograms very cheaply on the few columns where they are most likely to have a beneficial effect.
I’ve written about optimizer defects with descending indexes before now but a problem came up on the OTN database forum a few days ago that made me decide to look very closely at an example where the arithmetic was clearly defective. The problem revolves around a table with two indexes, one on a date column (TH_UPDATE_TIMESTAMP) and the other a compound index which starts with the same column in descending order (TH_UPDATE_TIMESTAMP DESC, TH_TXN_CODE). The optimizer was picking the “descending” index in cases where it was clearly the wrong index (even after the statistics had been refreshed and all the usual errors relating to date-based indexes had been discounted). Here’s an execution plan from the system which shows that there’s something wrong with the optimizer:
One thing you (ought to) learn very early on in an Oracle career is that there are always cases you haven’t previously considered. It’s a feature that is frequently the downfall of “I found it on the internet” SQL. Here’s one (heavily paraphrased) example that appeared on the OTN database forum a few days ago:
select table_name,round((blocks*8),2)||’kb’ “size” from user_tables where table_name = ‘MYTABLE';
select table_name,round((num_rows*avg_row_len/1024),2)||’kb’ “size” from user_tables where table_name = ‘MYTABLE';
The result from the first query is 704 kb, the result from the second is 25.4 kb … fragmentation, rebuild, CTAS etc. etc.
That’s data that isn’t there until you look for it, sort of, from the optimizer’s perspective.
Here’s some code to create a sample data set:
create table t1 as with generator as ( select --+ materialize rownum id from dual connect by level <= 1e4 ) select rownum id, mod(rownum-1,200) mod_200, mod(rownum-1,10000) mod_10000, lpad(rownum,50) padding from generator v1, generator v2 where rownum <= 1e6 ; begin dbms_stats.gather_table_stats( ownname => user, tabname =>'T1', method_opt => 'for all columns size 1' ); end; /
Now derive the execution plans for a couple of queries noting, particularly, that we are using queries that are NOT CONSISTENT with the current state of the data (or more importantly the statistics about the data) – we’re querying outside the known range.
“You can’t compare apples with oranges.”
Oh, yes you can! The answer is 72,731,533,037,581,000,000,000,000,000,000,000.
Here’s a question that appeared in my email a few days ago:
Based on the formula: “sreadtim = ioseektim + db_block_size/iotrfrspeed” sreadtim should always bigger than ioseektim.
But I just did a query on my system, find it otherwise, get confused,
This is a quick response to a question on an old blog post asking how you can adjust the high value if you’ve already got a height-balanced histogram in place. It’s possible that someone will come up with a tidier method, but this was just a quick sample I created and tested on 22.214.171.124 in a few minutes. (Note – this is specifically for height-balanced histograms, and it’s not appropriate for 12c which has introduced hybrid histograms that will require me to modify my “histogram faking” code a little).
Like the recent article on deleting histograms this is another draft that I rediscovered while searching for some notes I had written on a different topic – so I’ve finally finished it off and published it.
Here’s a quirky little detail of extended stats that came up in an OTN thread earlier on this week [ed: actually 8th Jan 2014]. When you create column group stats, Oracle uses an undocumented function sys_op_combined_hash() to create a hash value, and if you gather simple stats on the column (i.e. no histogram) you can get some idea of the range of values that Oracle generates through the hash function. For example:
Here’s a note which I drafted in Novemeber 2010, and then didn’t publish. I found it earlier on this morning while looking for another note I’d written about histograms so, even though it may not be something that people need so much these days, I thought: better late than never.
I’ve pointed out in the past that I’m not keen on seeing lots of histograms on a system and tend to delete them if I think they are not needed. Here’s an example of the type of code I use to delete a histogram.