Column Stats

A little while ago I added a post-script to a note I’d written five years ago about gathering stats on a virtual column and had updated with a reference to a problem on the Oracle database forum that complained that stats collection had taken much longer after the addition of a function-based index. The problem related to the fact that the function-based index was supported by a virtual column that used an instr() function on (more...)

Stats time

I wrote a note a couple of years ago explaining how I used to get a rough idea (with some errors) of how much time was spent in the overnight stats collection by each object. One of the nice little enhancements that appeared in 12c was the appearance of a couple of functions that can report information about this type of thing, and more. These are the dbms_stats function report_stats_operations() and report_single_stats_operation() with the following (more...)

Subquery Order

From time to time I’ve wanted to optimize a query by forcing Oracle to execute existence (or non-existence) subqueries in the correct order because I know which subquery will eliminate most data most efficiently, and it’s always a good idea to look for ways to eliminate early. I’ve only just discovered (which doing some tests on 18c) that Oracle 12.2.0.1 introduced the /*+ order_subq() */ hint that seems to be engineered to (more...)

RTFM ?

My entrance at the Polish Oracle User Group conference 2017 has just resurfaced on Twitter. There is a back-story to this which contains an allegorical lesson in using Oracle. As I said in the opening remarks in the subsequent presentation: “The lesson began before I got to the stage”.

Like all the other speakers at POUG2017 I had received an email asking me to name a tune for a playlist. Having decided that Beethoven, Brahms (more...)

Error Logging

Error logging is a topic that I’ve mentioned a couple of times in the past, most recently as a follow-up in a discussion of the choices for copying a large volume of data from one table to another, but originally in an addendum about a little surprise you may get when you use extended strings (max_string_size = EXTENDED).

If you use the default call to dbms_errlog.create_error_log() to create an error logging table (more...)

Descending bug

Following on from Monday’s posting about reading execution plans and related information, I noticed a question on the ODC database forum asking about the difference between “in ({list of values})” and a list of “column = {constant}” predicates connected by OR. The answer to the question is that there’s essentially no difference as you would be able to see from the predicate section of an execution plan:


SELECT  c1, c2, c3, c4, c5, c6, c7,  (more...)

Masterclass – 1

A recent thread on the Oracle developer community database forum raised a fairly typical question with a little twist. The basic question is “why is this (very simple) query slow on one system when it’s much faster on another?” The little twist was that the OP was using “Streams Replication” to replicate data between the two systems.

To make life easy for remote trouble-shooters, the OP has supplied for each system the output from (more...)

Extended Histograms – 2

Following on from the previous posting which raised the idea of faking a frequency histogram for a column group (extended stats), this is just a brief demonstration of how you can do this. It’s really only a minor variation of something I’ve published before, but it shows how you can use a query to generate a set of values for the histogram and it pulls in a detail about how Oracle generates and stores column (more...)

Extended Histograms

Today’s little puzzle comes courtesy of the Oracle-L mailing list. A table has two columns (c2 and c3), which contain only the values ‘Y’ and ‘N’, with the following distribution:


select   c2, c3, count(*)
from     t1
group by c2, c3
;

C C   COUNT(*)
- - ----------
N Y       1994
Y N      71482

2 rows selected.

The puzzle is this – how do you get the optimizer to predicat a cardinality of (more...)

Direct IOT

A recent (automatic ?) tweet from Connor McDonald highlighted an article he’d written a couple of years ago about an enhancement introduced in 12c that allowed for direct path inserts to index organized tables (IOTs). The article included a demonstration seemed to suggest that direct path loads to IOTs were of no benefit, and ended with the comment (which could be applied to any Oracle feature): “Direct mode insert is a very cool facility, but (more...)