SQL Query Performance

pzozulka
pzozulka used Ask the Experts™
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A few questions on SQL performance. Query will be run against millions of records so performance is key.

1. If I have the following condition: where t1.date < getdate()

Would it improve performance to run getdate() once at the beginning of the query, and save it to a variable? Then do: where t1.date < variable? This way each record is checked against a variable instead of having to run getdate() a million times. I'm assuming it's run for every record -- one record at a time? At least that's how I think SQL parses queries.

2. Does it matter what order the conditions are placed in the where clause? For example, certain conditions eliminate 50% of all records. Should such conditions be placed at the beginning of the where clause, and conditions that eliminate only 5% of records placed towards the end?
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SQL Server Data Dude
Most Valuable Expert 2013
Author of the Year 2015
Commented:
One GETDATE() I believe is okay, but multiple GETDATE() could result in one execution for each row.   Multiple GETDATE()'s are still fine, but you run the risk of different values.

If you use a function though, like DATEDIFF(d, -1, GETDATE()), then it would process once for every row.

The variable route..

Declare @dt datetime = GETDATE()
SELECT blah, blah, blah FROM your_table WHERE some_date_column < @dt

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>2. Does it matter what order the conditions are placed in the where clause?
No, unless you have parentheses marks ( ) to explicitly state order of execution.
Scott PletcherSenior DBA
Most Valuable Expert 2018
Top Expert 2014
Commented:
1.  No, the query plan should be the same.  As I understand it, SQL will generate only a single GETDATE() value for all uses of GETDATE() in the same batch.

Edit: Far more important is the data type of the table column.  To prevent unwanted implicit conversions of the data column, you'd be better off converting the GETDATE() to the relevant character string.

2.  Generally no, although it can't hurt to put the more-restricting checks first.  Within a CASE WHEN, however, the clauses are always evaluated in order, so order would of course matter then.
David ToddSenior Database Administrator
Commented:
Hi,

Just very occasionally, on older versions of SQL, I found it was beneficial to use derived tables in the from clause, and put as many conditions as possible in that subquery - particularly if the base table was large.) Or even return that first query to a temp table, if necessary add indexes, and then perform the rest of the query.

HTH
  David

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