Indexing

Jonathan Lewis's picture

Min/Max

One of my most-repeated observations about trouble-shooting Oracle is that things break when you start combining features. Here’s an example that demonstrates the point.

It’s possible to create “descending” indexes – or indexes with descending columns, as I prefer to call them, and there’s a special “min/max range scan” optimizer operation for a particular kind of index usage – demonstrated in the following code fragment (running under 11.2.0.4, and reporting the rowsource execution statistics):

Jonathan Lewis's picture

Min/Max

One of my most-repeated observations about trouble-shooting Oracle is that things break when you start combining features. Here’s an example that demonstrates the point.

It’s possible to create “descending” indexes – or indexes with descending columns, as I prefer to call them, and there’s a special “min/max range scan” optimizer operation for a particular kind of index usage – demonstrated in the following code fragment (running under 11.2.0.4, and reporting the rowsource execution statistics):

Jonathan Lewis's picture

Shrink Space

Here’s a lovely effect looking at v$lock (on 11.2.0.4)

Jonathan Lewis's picture

Auto Sample Size

In the past I have enthused mightily about the benefits of the approximate NDV mechanism and the benefit of using auto_sample_size to collect statistics in 11g; however, as so often happens with Oracle features, there’s a down-side or boundary condition, or edge case. I’ve already picked this up once as an addendum to an earlier blog note on virtual stats, which linked to an article on OTN describing how the time taken to collect stats on a table increased dramatically after the addition of an index – where the index had this definition:

Jonathan Lewis's picture

FBI Skip Scan

A recent posting on the OTN database forum highlighted a bug (or defect, or limitation) in the way that the optimizer handles index skip scans with “function-based” indexes – it doesn’t do them. The defect has probably been around for a long time and demonstrates a common problem with testing Oracle – it’s very easy for errors in the slightly unusual cases to be missed; it also demonstrates a general principle that it can take some time for a (small) new feature to be applied consistently across the board.

The index definitions in the original posting included expressions like substr(nls_lower(colX), 1, 25), and it’s possible for all sorts of unexpected effects to appear when your code starts running into NLS  settings, so I’ve created a much simpler example. Here’s my table definition, with three index definitions:

Jonathan Lewis's picture

Indexing LOBs

Many years ago, possibly when most sites were still using Oracle 8i, a possible solution to a particular customer problem was to create a function-based index on a CLOB column using the dbms_lob.getlength() function call. I can’t find the notes explaining why this was necessary (I usually have some sort of clue – such as the client name – in the script, but in this case all I had was a comment that “the manuals say you can’t do this, but it works provided you wrap the dbms_lob call inside a deterministic function”).

Jonathan Lewis's picture

Index Compression – aargh

The problem with telling people that some feature of Oracle is a “good thing” is that some of those people will go ahead and use it; and if enough people use it some of them will discover a hitherto undiscovered defect. Almost inevitably the bug will turn out to be one of those “combinations” bugs that leaves you thinking: “Why the {insert preferred expression of disbelief here} should {feature X} have anything to do with {feature Y}”.

Here – based on index compression, as you may have guessed from the title – is one such bug. I got it first on 11.1.0.7, but it’s still there on 11.2.0.4 and 12.1.0.1

Jonathan Lewis's picture

Quiz

To create an index on a table (with no existing indexes) Oracle has to start by doing a tablescan.

What’s the difference between the tablescan it uses for a B-tree index and the tablescan it uses for a bitmap index ? Why ?

Update:

I was going to give a hint that if you answered the “why” first that might lead you to the right idea and a test for the “what”, but we already have an answer, with a sample of proof.

Jonathan Lewis's picture

Modify PK

Sitting in the lounge waiting to be called for my flight I was musing on the 12c feature of having multiple indexes defined on the same ordered column set  when a thought crossed my mind and I decided to run a little test that looked like this:

Jonathan Lewis's picture

Pagination

I was involved in a thread on Oracle-L recently started with the question: “How many LIOs is too many LIOs”. Rather than rewrite the whole story, I’ve supplied a list of links to the contributions I made, in order – the final “answer” is actually the answer to a different question – but travels an interesting path to get there.#

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