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Undo Understood

It’s hard to understand all the ramifications of Oracle’s undo handling, and it’s not hard to find cases where the resulting effects are very confusing. In a recent post on the OTN database forum resulted in one response insisting that the OP was obviously updating a table with frequent commits from one session while querying it from another thereby generating a large number of undo reads in the querying session.

Join Elimination

A question has just appeared on OTN describing a problem where code that works in 11g doesn’t work in 12c (exact versions not specified). The code in question is a C-based wrapper for some SQL, and the problem is a buffer overflow problem. The query supplied is as follows:

select T1.C1 from T1, T2 where T1.C1 = T2.D1;

The problem is that this works in 11g where the receiving (C) variable is declared as

char myBuffer [31];

but it doesn’t work in 12c unless the receiving variable is declared as:


A recent posting on the OTN database forum described a problem with an insert (as select) statement that sometimes ran extremely slowly: nothing interesting yet, there could be plenty of boring reasons for that to happen. The same SQL statement (by SQL_ID) might take 6 hours to insert 300K rows one night while taking just a few minutes to insert 900K another night (still nothing terribly interesting).

Truncate 12c

Here’s one of those little improvements in 12c (including 12.1) that will probably end up being described as “little known features” in about 3 years time. Arguably it’s one of those little things that no-one should care about because it’s not the sort of thing you should do on a production system, but that doesn’t mean it won’t be seen in the wild.

Rather than simply state the feature I’m going to demonstrate it, starting with a little code to build a couple of tables with referential integrity:

Basicfile LOBs

I wrote a short series a little while ago about some of the nasty things that can happen (and can’t really be avoided) with Basicfile LOBs and recently realised that it needed a directory entry so that I didn’t have to supply 6 URLs if I wanted to point someone to it; so here’s the catalogue:

IOT limitation

In the right circumstances Index Organized Tables (IOTs) give us tremendous benefits – provided you use them in the ideal fashion. Like so many features in Oracle, though, you often have to compromise between the benefit you really need and the cost of the side effect that a feature produces.

Securefile space

Here’s a little script I 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:

Basicfile LOBs 6

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 need when things behave strangely, so in this installment of my series I’m going to take about an example with with storage enabled in row.

Basicfile LOBS 5

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 that they should have been re-used. Our LOB segment started at 8,192 blocks, is currently at 8,576 blocks and is only using 8,000 of them.

How will things look if I now connect a new session (which might be associated with a different freepool), delete the oldest 3,000 LOBs, wait a little while, then get my original four sessions to do their concurrent inserts again ? And what will things look like after I’ve repeated this cycle several times ?

Space Usage

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.