One of the little myths of Oracle appeared on the Oracle-L list server a few days ago – the one that says: “you don’t need a histogram on a single column unique/primary key”.
Not only can a histogram be helpful on a column that’s declared to hold unique values, the optimizer may even spot the need automatically. It’s a little unusual (and probably the result of poor programming practice) but it does happen. Here’s an (more...)
This note comes to you prompted by “Noons” in a recent twitter exchange
In response to a complaint by Lukas Eder about having to educate people in 2016 that there is no (performance) difference between count(*) and count(1), Nuno asked me to blog about my claim that this non-difference is a good educational example on at least three different counts.
One thing I won’t count in the list of three is an actual demonstration (more...)
Yesterday I posted a note about querying dba_optstat_operations to get a quick report of how long calls to dbms_stats had been taking but said I had another script that helped to fill some of the gaps it left. One of my readers points out fairly promptely that 12c enhances the feature considerably, with a view dba_optstat_operation_tasks that (for example) lists all the tables processed during a single call to gather_schema_stats.
Well, I wrote my script (more...)
I don’t really remember how long it’s been since Oracle created an automatic log of how long a call to the dbms_stats package took, though it was probably some time in the 10g time-line. It wasn’t until it had been around for several years, though before I wrote little script (possibly prompted by a comment from Martin Widlake) that I’ve used occasionally since to see what’s been going on in the past, how variable (more...)
A script hacked together a couple of years ago from a clone of a script I’d been using for checking space usage in the older types of segments. Oracle Corp. eventually put together a routine to peer inside securefile LOBs:
rem Script: dbms_space_use_sf.sql
rem Author: Jonathan Lewis
rem Dated: Dec 2013
rem Last tested
rem Not tested
rem 11.1. (more...)
One of the nice things about declaring your (basicfile) LOBs as “enable storage in row” is that the block addresses of the first 12 chunks will be listed in the row and won’t use the LOB index, so if your LOBs are larger than 3960 bytes but otherwise rather small the LOB index will hold only the timestamp entries for deleted LOBs. This makes it just a little easier to pick out the information you (more...)
At the end of the last installment we had seen a test case that caused Oracle to add a couple of redundant new extents to a LOB segment after one process deleted 3,000 LOBs and another four concurrent processes inserted 750 LOBs each a few minutes later (after the undo retention period had elapsed). To add confusion the LOBINDEX seemed to show that all the “reusable” chunks had been removed from the index which suggests (more...)
Here’s a simple script that I’ve used for many years to check space usage inside segments. The comment about freelist groups may be out of date – I’ve not had to worry about that for a very long time. There is a separate script for securefile lobs.
rem Script: dbms_space_use.sql
rem Author: Jonathan Lewis
rem Dated: Nov 2002
rem Last tested
rem 11.2. (more...)
Prompted by a recent OTN posting I’ve dug out from my library the following demonstration of an anomalty with the parallel_index() hint. This note is a warning about how little we understand hints and what they’re supposed to mean, and how we can be caught out by an upgrade. We’ll start with a data set which, to match a comment made in the origina posting rather than being a necessity for the demonstration, has an (more...)
At the end of the previous installment we saw that a single big batch delete would (apparently) attach all the “reusable” chunks into a single freepool, and asked the questions:
- Why would the Oracle developer think that this use of one freepool is a good idea ?
- Why might it be a bad idea ?
- What happens when we start inserting more data ?
(Okay, I’ll admit it, the third question is a clue about the answer (more...)