opt_estimate 5

If you’ve been wondering why I resurrected my drafts on the opt_estimate() hint, a few weeks ago I received an email containing an example of a query where a couple of opt_estimate() hints were simply not working. The critical features of the example was that the basic structure of the query was of a type that I had not previously examined. That’s actually a common type of problem when trying to investigate any Oracle feature (more...)

Assumptions

Over the last few days I’ve been tweeting little extracts from Practical Oracle 8i, and one of the tweets contained the following quote:

This lead to the (more...)

DB links

A recent thread on the ODC SQL & PL/SQL forum raised the question of how data moves across a database link – is it row by row or array based (or other)? It’s a good question as it’s fairly common knowledge that distributed queries can be very much slower than the same query when executed on an equivalent set of local tables, so it’s worth having a little poke around to see if there’s anything (more...)

opt_estimate 4

In the previous article in this series on the opt_estimate() hint I mentioned the “query_block” option for the hint. If you can identify a specify query block that becomes an “outline_leaf” in an execution plan (perhaps because you’ve deliberately given an query block name to an inline subquery and applied the no_merge() hint to it) then you can use the opt_estimate() hint to tell the optimizer how many rows will be produced by that query (more...)

opt_estimate 3

This is just a quick note to throw out a couple of of the lesser-known options for the opt_estimate() hint – and they may be variants that are likely to be most useful since they address a problem where the optimizer can produce consistently bad cardinality estimates. The first is the “group by” option – a hint that I once would have called a “strategic” hint but which more properly ought to be called a (more...)

Glitches

Here’s a question just in from Oracle-L that demonstrates the pain of assuming things work consistently when sometimes Oracle development hasn’t quite finished a bug fix or enhancement. Here’s the problem – which starts from the “scott.emp” table (which I’m not going to create in the code below):

rem
rem     Script:         fbi_fetch_first_bug.sql
rem     Author:         Jonathan Lewis
rem     Dated:          June 2019
rem 

-- create and populate EMP table from SCOTT demo schema

create index  (more...)

opt_estimate 2

This is a note that was supposed to be a follow-up to an initial example of using the opt_estimate() hint to manipulate the optimizer’s statistical understanding of how much data it would access and (implicitly) how much difference that would make to the resource usage. Instead, two years later, here’s part two – on using opt_estimate() with nested loop joins. As usual I’ll start with a little data set:


rem
rem     Script:         opt_est_nlj.sql
rem      (more...)

ANSI bug

The following note is about a script that I found on my laptop while I was searching for some details about a bug that appears when you write SQL using the ANSI style format rather than traditional Oracle style. The script is clearly one that I must have cut and pasted from somewhere (possibly the OTN/ODC database forum) many years ago without making any notes about its source or resolution. All I can say about (more...)

Can’t Unnest

In an echo of a very old “conditional SQL” posting, a recent posting on the ODC general database discussion forum ran into a few classic errors of trouble-shooting. By a lucky coincidence this allowed me to rediscover and publish an old example of parallel execution gone wild before moving on to talk about the fundamental problem exhibited in the latest query.

The ODC thread started with a question along the lines of “why isn’t Oracle (more...)

Trouble-shooting

Here’s an answer I’ve just offered on the ODC database forum to a fairly generic type of problem.

The question was about finding out why a “program” that used to take only 10 minutes to complete is currently taking significantly longer. The system is running Standard Edition, and the program runs once per day. There’s some emphasis on the desirability of taking action while the program is still running with the following as the most (more...)