Oracle Scratchpad

May 9, 2013

Clustering_factor

Filed under: CBO,Indexing,Oracle — Jonathan Lewis @ 8:14 am BST May 9,2013

Cost Based Oracle – Fundamentals (November 2005)

But the most interesting function for our purposes is sys_op_countchg(). Judging from its name, this function is probably counting changes, and the first input parameter is the block ID portion (object_id, relative file number, and block number) of the table’s rowid, so the function is clearly matching our notional description of how the clustering_factor is calculated. But what is that 1 we see as the second parameter?

When I first understood how the clustering_factor was defined, I soon realized that its biggest flaw was that Oracle wasn’t remembering recent history as it walked the index; it only remembered the previous table block so that it could check whether the latest row was in the same table block as last time or in a new table block. So when I saw this function, my first guess (or hope) was that the second parameter was a method of telling Oracle to remember a list of previous block visits as it walked the index.

And finally, Oracle Corp. had implemented an official interface to the second parameter of sys_op_countchg() – provided you install the right patch – through a new table (or schema, or database) preference type available to the dbms_stats.set_table_prefs() procedure.

I’ve been meaning to write this post for two or three months, ever since Sean Molloy sent me an email about short blog note from Martin Decker describing Bug 13262857  Enh: provide some control over DBMS_STATS index clustering factor computation. Unfortunately I’ve not yet had time to investigate the patch, but I don’t think I need to any more because Richard Foote has written it up in his latest blog post.

Read Richard’s post – it’s important.

Update 10th May

Richard’s post has, unsurprisingly, produced a buzz of excitement among his readers – and started up the discussion of how best to use this capability; so here’s another quote from the book (p.111 – available in the download of chapter 5):

So using Oracle’s own function for calculating the clustering_factor, but substituting the freelists value for the table, may be a valid method for correcting some errors in the clustering_factor for indexes on strongly sequenced data. (The same strategy applies if you use multiple freelist groups—but multiply freelists by freelist groups to set the second parameter.)

Can a similar strategy be used to find a modified clustering_factor in other circumstances? I think the answer is a cautious “yes” for tables that are in ASSM tablespaces. Remember that Oracle currently allocates and formats 16 new blocks at a time when using automatic segment space management (even when the extent sizes are very large, apparently). This means that new data will be roughly scattered across groups of 16 blocks, rather than being tightly packed.

Calling Oracle’s sys_op_countchg() function with a parameter of 16 could be enough to produce a reasonable clustering_factor where Oracle currently produces a meaningless one. The value 16 should, however, be used as an upper bound. If your real degree of concurrency is typically less than 16, then your actual degree of concurrency would probably be more appropriate.

Whatever you do when experimenting with this function—don’t simply apply it across the board to all indexes, or even all indexes on a particular table. There will probably be just a handful of critical indexes where it is a good way of telling Oracle a little more of the truth about your system—in other cases you will simply be confusing the issue.

Note particularly the comments about how the best value depends on the data in the indexed columns, the table configuration, and the degree of concurrency – you don’t necessarily want to use the same value for every index on a given table. That’s a shame, since Oracle has defined the interface as a TABLE preference, so if you set it then you get the same for every index on that table. Despite this, if you’re prepared to put in a little control work, it does mean that you can use an official Oracle mechanism to play the game I was suggesting in the book – for each “special” index, set the preference, collect the stats, then clear the preference.

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