300,00,00 row tablescan

From: <jonathan_at_jlcomp.demon.co.uk>
Date: Thu, 01 May 2003 19:31:52 -0800
Message-ID: <F001.0058DD7E.20030501193152@fatcity.com>

Well, I really enjoyed IOUG-A,
but now that it's over, I can reveal
how to convince Oracle 9.2 that a
300,000,000 row tablescan is cheaper
than a primary key index unique scan.

Many of you realised that the first
step was optimizer_index_cost_adj = 10000. This made the index look very expensive.

Many of you also hit on the idea of creating the index in a tablespace with a 2K block size to squeeze an extra branch level, and add that little (100) extra cost to the index.

After that, people got a bit stuck, although setting db_file_multiblock_read_count was a common suggestion.

One little fix, though, was to set:
_table_scan_cost_plus_one = false,
to make tablescans a little cheaper.
Then I told Oracle that I had a 32 GHz
CPU, and a disk capable of 5GB/s throughput by using dbms_stats.set_system_stats. And Oracle believed me:

begin

  dbms_stats.set_system_stats('MBRc',32768);
  dbms_stats.set_system_stats('CPUSPEED',32768);
  dbms_stats.set_system_stats('SREADTIM',10);

  dbms-stats.set_system_stats('MREADTIM',10.01); ed;

So Oracle decided that a massive tablescan was physically cheaper than an index scan, and that the CPU cost of checking all those rows was irelevant.

QED Jonathan Lewis.

Now booking for Tutorials pre/post
Oracle World - SF, Sept 2003.  

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Author:
  INET: jonathan_at_jlcomp.demon.co.uk

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