Thursday, June 04, 2009

What is sort?

Inspired by discussion about sorting (PLAN SORT), did some simple tests. Right now I do not have "an article" about this, but want to show you some discovered facts:
  • sorting the temporary file (fb_sort_nnnn.tmp and ib_sort_nnnn.tmp) mostly does writes, not reads (excluding database reads and fetching data from temp file). Firebird 2.1 read/write ratio is 1:10. InterBase 2009 read/write ratio is different, and nearly 2/3.
  • turning on Windows folder/files compression for TEMP lowers sorting temp file size up to 2-4 times (depends on sorting data, I've used repeating data, sorry)
  • turning on Windows folder/files compression for TEMP increases processor load 2 times, and makes disk transfer ~4 times less.
  • Firebird 2.1 show only small (invaluable) diffrerence when sorting at compressed and uncompressed TEMP (4 min 00 sec). Compressed TEMP produces more stable timings, when test is run several times.
  • InterBase 2009 sorting speed is equal to Firebird 2.1 only on uncompressed TEMP. And it is slower at compressed TEMP (4 min 00 sec vs 5 min 00 sec). The cause is InterBase's higher read/write ratio (uncompression of blocks being read)
  • on my computer sorting 31 million records (select varchar(20) from ...) produces temp file with the size of 4.28GB (uncompressed)
  • InterBase 2009 uses bigger sorting blocks (chunks) than Firebird 2.1. By "sorting block" I mean set of records that are being sorted in memory and then written to sort file for future merge with other blocks
  • Using uncompressed TEMP InterBase 2009 loads processor less than Firebird 2.1 (35% vs 40%), but writes to disk faster (30mb/sec vs 25mb/sec)
This was a "single-user" test. Running concurrent sorting queries may produce different result. What can be said now for sure, is that if you have lot of queries with PLAN SORT, you must (!) have TEMP pointing to the separate physical drive. And maybe RAID 0 will help.
So, questions? :-)

1 comment:

Milan Babuskov said...

Hi Dmitry,

The best results you can get is when sorting is done in memory. With Firebird 2.0 or lower you can create a RAM disk and set TempDirectory in firebird.conf to use it. With 2.1 this is done automatically - just increase the setting TempCacheLimit which has a really low default setting.