Can anyone help with my understanding of the processing score reported by EXPLAIN and the way it relates (or doesn't) to processing time? I test-run queries against an old, slow linux box in order to help spot time or processor consuming queries, because I don't understand the figures that EXPLAIN returns :( example: explain select a.field1, b.field1 from table1 a join table2 b on field a.field2=b.field2 limit 100; sequential scan on table2 rows 12900 width 30 cost 0.00..249 (actual elapsed time 3.4 secs) example: explain select a.field1, b.field1 from table1 a join table2 b on field a.field2=b.field2 where b.field1='xxx' limit 100; index scan on table2 using index_xxx rows 26 width 30 cost 0.00..23 (actual elapsed time 6.4 secs) Why is the actual elapsed time higher for the second example? Is the "figure of merit" given by explain attempting to describe the relative processing requirement to run the query, or is it (as I suspect) the requirement to setup the structure of the query, ready to actually do the work - and the work processing requirement will vary dependant upon the data, memory available to perform match filtering etc? I would like to understand this better, as otherwise the only real means I can think of for tuning queries in order to minimise processing requirements is the long winded reported query time tests based on example queries. -- Best regards, Andy mailto:[EMAIL PROTECTED]
