Question

I have this query:

SELECT *
FROM sample
   INNER JOIN test ON sample.sample_number = test.sample_number
   INNER JOIN result ON test.test_number = result.test_number
   WHERE sampled_date BETWEEN '2010-03-17 09:00' AND '2010-03-17 12:00'

the biggest table here is RESULT, contains 11.1M records. The left 2 tables about 1M.

this query works slowly (more than 10 minutes) and returns about 800 records. executing plan shows clustered index scan (over it’s PRIMARY KEY (result.result_number, which actually doesn’t take part in query)) over all 11M records.
RESULT.TEST_NUMBER is a clustered primary key.

if I change 2010-03-17 09:00 to 2010-03-17 10:00 – i get about 40 records. it executes for 300ms. and plan shows index seek (over result.test_number index)

if i replace * in SELECT clause to result.test_number (covered with index) – then all become fast in first case too. this points to hdd IO issues, but doesn’t clarifies changing plan.

so, any ideas?

UPDATE:
sampled_date is in table sample and covered by index.
other fields from this query: test.sample_number is covered by index and result.test_number too.

UPDATE 2:
obviously than sql server in any reasons don’t want to use index.

i did a small experiment: i remove INNER JOIN with result, select all test.test_number and after that do

SELECT * FROM RESULT WHERE TEST_NUMBER IN (...)

this, of course, works fast. but i cannot get what is the difference and why query optimizer choose such inappropriate way to select data in 1st case.

UPDATE 3:
after backing up database and restoring to database with new name – both requests work fast as expected even on much more ranges…

so – are there any special commands to clean or optimize, whatever, that can be relevant to this? 🙁

Answer 1

A couple things to try:

• Update statistics
• Add hints to the query about what index to use (in SQL Server you might say WITH (INDEX(myindex)) after specifying a table)

EDIT: You noted that copying the database made it work, which tells me that the index statistics were out of date. You can update them with something like UPDATE STATISTICS mytable on a regular basis.

Use EXEC sp_updatestats to update the whole database.