Jonathan Lewis's picture

Wrong Results ?

I gather that journalistic style dictates that if the headline is a question then the answer is no. So, following on from a discussion of possible side effects of partition exchange, let’s look at an example which doesn’t involve partitions.  I’ve got a schema that holds nothing by two small, simple heap tables, parent and child, (with declared primary keys and the obvious referential integrity constraint) and I run a couple of very similar queries that produce remarkably different results:

Jonathan Lewis's picture

Partition Limit

A tweet from Connor McDonald earlier on today reminded me of a problem I managed to pre-empt a couple of years ago.

Partitioning is wonderful if done properly but it’s easy to get a little carried away and really foul things up. So company “X” decided they were going to use range/hash composite partitioning and, to minimise contention and (possibly) reduce the indexing overheads, they decided that they would create daily partitions with 1,024 subpartitions.

This, in testing, worked very well, and the idea of daily/1024 didn’t seem too extreme given the huge volume of data they were expecting to handle. There was, however, something they forgot to test; and I can demonstrate this on 12c with an interval/hash partitioned table:

Jonathan Lewis's picture

Quiz Night

I was setting up a few tests on a copy of recently when I made a mistake creating the table – I forgot to put in a couple of CAST() calls in the select list, so I just patched things up with a couple of “modify column” commands. Since I was planning to smash the table in all sorts of ways and it had taken me several minutes to create the data set (10 million rows) I decided to create a clean copy of the data so that I could just drop the original table and copy back the clean version – and after I’d done this I noticed something a little odd.

Here’s the code (cut down to just 10,000 rows), with a little output:

Jonathan Lewis's picture

Partitioned Bitmap Join

If you don’t want to read the story, the summary for this article is:

If you create bitmap join indexes on a partitioned table and you use partition exchanges to load data into the table then make sure you create the bitmap join indexes on the loading tables in exactly the same order as you created them on the partitioned table or the exchange will fail with the (truthful not quite complete) error: ORA-14098: index mismatch for tables in ALTER TABLE EXCHANGE PARTITION.

Jonathan Lewis's picture

Bitmap Efficiency

An interesting observation came up on the Oracle-L list server a few days ago that demonstrated how clever the Oracle software is at minimising run-time work, and how easy it is to think you know what an execution plan means when you haven’t actually thought through the details – and the details might make a difference to performance.

The original question was about a very large table with several bitmap indexes, and an anomaly that appeared as a query changed its execution plan.  Here are the critical sections from the plans (extracted from memory with rowsource execution statistics enabled):

Jonathan Lewis's picture

Drop Column

I published a note on AllthingsOracle a few days ago discussing the options for dropping a column from an existing table. In a little teaser to a future article I pointed out that dropping columns DOESN’T reclaim space; or rather, probably doesn’t, and even if it did you probably won’t like the way it does it.

I will  be writing about “massive deletes” for AllthingsOracle in the near future, but I thought I’d expand on the comment about not reclaiming space straight away. The key point is this – when you drop a column you are probably dropping a small fraction of each row. (Obviously there are some extreme variants on the idea – for example, you might have decided to move a large varchar2() to a separate table with shared primary key).

Jonathan Lewis's picture

Nul points

(To understand the title, see this Wikipedia entry)

The title could also be: “Do as I say, don’t do as I do”, because I want to remind you of an error that I regularly commit in my demonstrations. Here’s an example:

SQL> create table t (n number); 

Table created 

Have you spotted the error yet ? Perhaps this will help:

Jonathan Lewis's picture

Read Consistency

I posted a note a few days ago about read consistency, the Cross Session PL/SQL Function Result Cache, deterministic functions, and scalar subqueries. The intent of the article was to make clear the point that while you might think that declaring a PL/SQL function to be deterministic or in the PL/SQL Result Cache might make a query that calls the function perform faster, if that function contained its own SQL statement then your code might not be producing self-consistent results and (even worse) if you had used the Result Cache option your code might actually cause other session to get wrong results if you tried to “set transaction read only” or “alter session set isolation_level = serializable”

Jonathan Lewis's picture

Result Cache

Yesterday I thought I’d spend half an hour before breakfast creating a little demonstration of a feature; some time about midnight I felt it was time to stop because I’d spent enough time chasing around a couple of bugs that produced wrong results in a variety of ways. Today’s short post is just little warning: be VERY careful what you do with the PL/SQL result cache – if you use the results of database queries in the cache you may end up with inconsistent results in your application. Here’s one very simple example of what can go wrong, starting with a little script:

Jonathan Lewis's picture

Truncate – 2

Following on from my earlier comments about how a truncate works in Oracle, the second oldest question about truncate (and other DDL) appeared on the OTN database forum“Why isn’t a commit required for DDL?”

Sometimes the answer to “Why” is simply “that’s just the way it is” – and that’s what it is in this case, I think.  There may have been some historic reason why Oracle Corp. implemented DDL the way they did (commit any existing transaction the session is running, then auto-commit when complete), but once the code has been around for a few years – and accumulated lots of variations – it can be very difficult to change a historic decision, no matter how silly it may now seem.

Syndicate content