When a “cache read” tablescan (or index fast full scan) takes place we generally expect to see waits on “db file scattered read” as Oracle performs multi-block reads to do the scan. But we all know that Oracle will skip over blocks that are already in the cache and can therefore end up doing multi-block reads of many different sizes, even down to the point where it does single block reads (waiting for “db file sequential read”).
A quirky little question came up on OTN a little while ago: “for a large table we expect multiblock reads to be positioned at the end of the LRU for prompt re-use; but when Oracle does a single block read as part of a tablescan does it go to the end of the LRU (because it’s part of a large tablescan) or does it go to the mid-point of the LRU (because it’s a single block read)?”
The description of how blocks are treated in a tablescan has been simplified, of course, but the question is still valid – so what’s the answer, and how (without going into an extreme level of detail) would you demonstrate it ?
I see Tanel has just started a short series of articles on where the X$ data come from so, for those with time on their hands (?anyone?) here’s a little script I wrote a few years ago when I wanted to check which v$ objects corresponded to the same internal structure in the SGA: (as v$session and v$session_wait used to)
It starts with a function that has to be created in the SYS schema – so no doing this on production systems. The function is a pipelined function so that we can treat its output like a table, which means I need to create an object type and a table type before creating the function. In the function I select each x$ name from the list of x$ names (x$kqfta) and for each table I construct a dynamic SQL statement selecting the first row from the table.
Over the versions of Oracle, various bugs have resulted in deadlocks and crashes when selecting a row from particular X$ objects, so there a short list of exceptions that has grown gradually over time. The code has some calls to dbms_output(), which you can choose to hide by setting serveroutput off. Depending on your system you may see a very large number of X$ with no data in them.
Everybody “knows” that when you do a tablescan of a table that it starts with two buffer gets on the segment header, though older versions (ca. 8i and earlier) of Oracle used to do 4 buffer gets on the segment header. The upshot of this is that many people still say that if you create a table and insert a single row then you’re going to get 3 buffer gets when you tablescan a table: two for the segment header and one for the data block:
So here’s a test, with the second set of autotrace stats which, for reasons I’ll describe shortly, may not be immediately reproducible on your system:
It’s always useful to collect baseline information – especially when it helps you notice that the baseline has moved in a way that might explain the next performance problem you see. Here’s an example demonstrating the benefit.
I have a table with a LOB column that is stored out of line. Many years ago I decided that I wanted to compare how the redo generation varied as I change the LOB from cached to nocache (with nologging). So here was one of my simplest test scripts (stripped to a minimum):
Just throwing out a brief comment (one of my many draft notes that I don’t have time to complete) about the dbms_space package. You’re probably familiar with this package and how, for ASSM segments, it can give you a measure of the available space in the blocks in a data segment, reporting 6 possible states of the blocks below the high high water mark (HHWM) e.g.:
This question came up on the OTN database forum a couple of months ago: “Why doesn’t Oracle allow you to create globally partitioned bitmap indexes?” The obvius answer is “It just doesn’t, okay.” But it can be quite interesting to think of reasons why a particular mechanism might not have been implemented – sometimes the answer can give you an insight into how a feature has been implemented, or it might suggest cases where a feature might not work very well, it might give you some ideas on how to work around a particular limitation, and sometimes it might just help to pass the time on a short flight.
Sometimes you get some questions on OTN lead to very geeky investigations. Here’s one that came up a while ago that started with a reasonable observation about recursive subquery factoring, then devolved into a real geek-attack question about buffer headers (x$bh) and buffer pins (x$kccbf).
I contributed a couple of ideas and some basic SQL to the discussion but never got around to doing anything concrete. If anyone has time and is sufficiently curious to play around I’d be interested to see what you did and what conclusions you came to.
I have, in the past, used the dbms_rowid package to create rowids from block addresses (typically faking the first and last rowids that could appear in an extent); but I’ve just been sent a piece of information by Valentin Nikotin that’s going to make me go back and check whether what I’ve done with the package will always give me the correct results. Here’s a little demonstration code that highlights the issue:
Oracle 12c has increased the maximum length of character-based columns to 32K bytes – don’t get too excited, they’re stored out of lines (so similar in cost to LOBs) and need some modification to the parameter file and data dictionary (starting the database in upgrade mode) before you can use them.
Richard Foote has a pair of articles on indexing such columns:
Be cautious about enabling this option and test carefully – there are going to be a number of side effects, and some of them may require a significant investment in time to resolve. The first one that came to my mind was that if you’ve created a function-based index on a pl/sql function that returns a varchar2() type and haven’t explicitly created the index on a substr() of the return value then the data type of the function’s return value will change from the current default of varchar2(4000) to varchar2(32767) – which means the index will become invalid and can’t be rebuilt or recreated.
Obviously you can redefine the index to include an explicit substr() call – but then you have to find all the code that was supposed to use the index and modify it accordingly.
Here’s a question that appeared recently on OTN, and it’s one I’ve wondered about a few times – but never spent any time investigating. Are there any overheads to enabling row movement on a table ? If not, why is it not enabled by default and eliminated as an option ?
Obviously there are costs when a row moves – it will be updated, deleted and re-inserted with all relevant index entries adjusted accordingly – but is there an inherent overhead even if you do nothing to move a single row ?
Equally obviously you’ve made it possible for some to “accidentally” shrink the table, cause short term locking problems, and longer term performance probems; similarly it becomes possible to update rows in partitioned tables in a way that causes them to move; but “someone might do it wrong” doesn’t really work as an argument for “de-featurising” something that need not have been a feature in the first place.
What have I missed ?
Answers in the comments gratefully received – and possibly discussed.
I wrote a note about the 12c “In-Memory” option some time ago on the OTN Database forum and thought I’d posted a link to it from the blog. If I have I can’t find it now so, to avoid losing it, here’s a copy of the comments I made:
I posted this question on twitter earlier on today (It was a thought that crossed my mind during a (terrible) presentation on partitioning that I had to sit through a few weeks ago – it’s always possible to be prompted to think of some interesting questions no matter how bad the presentation is, though):
Quiz: if you create a virtual column as trunc(date_col,’W’) and partition on it – will a query on date_col result in partition elimination?
A recent question on the Oracle-L list server described a problem with data coming in from SQL Server and an oddity with referential integrity failing on Oracle because (for example) a child row was in lower case while the parent was in upper.
This raised a few comments on how you might handle referential integrity while allowed case to differ. No doubt it’s been done before – by Tom Kyte if no-one else – but the first thought that crossed my mind was to use virtual columns:
I have a fairly strong preference for choosing simple solutions over complex solutions, and using Oracle-supplied packages over writing custom code – provided the difference in cost (whether that’s in human effort, run-time resources or licence fees) is acceptable. Sometimes, though, the gap between simplicity and cost is so extreme that a hand-crafted solution is clearly the better choice. Here’s an idea prompted by a recent visit to a site that makes use of materialized views and also happens to be licensed for the partitioning option.
Sorted Hash Clusters have been around for several years, but I’ve not yet seen them being used, or even investigated in detail. This is a bit of a shame, really, because they seem to be engineered to address a couple of interesting performance patterns.