By DForck42


2009-04-28 19:59:04 8 Comments

By definition (at least from what I've seen) sargable means that a query is capable of having the query engine optimize the execution plan that the query uses. I've tried looking up the answers, but there doesn't seem to be a lot on the subject matter. So the question is, what does or doesn't make an SQL query sargable? Any documentation would be greatly appreciated.

For reference: SARGable

4 comments

@user2011845 2017-06-09 21:08:21

For an operation to be considered sargable, it is not sufficient for it to just be able to use an existing index. In the example above, adding a function call against an indexed column in the where clause, would still most likely take some advantage of the defined index. It will "scan" aka retrieve all values from that column (index) and then eliminate the ones that do not match to the filter value provided. It is still not efficient enough for tables with high number of rows. What really defines sargability is the query ability to traverse the b-tree index using the binary search method that relies on half-set elimination for the sorted items array. In SQL, it would be displayed on the execution plan as a "index seek".

@BradC 2009-04-28 20:09:00

The most common thing that will make a query non-sargable is to include a field inside a function in the where clause:

SELECT ... FROM ...
WHERE Year(myDate) = 2008

The SQL optimizer can't use an index on myDate, even if one exists. It will literally have to evaluate this function for every row of the table. Much better to use:

WHERE myDate >= '01-01-2008' AND myDate < '01-01-2009'

Some other examples:

Bad: Select ... WHERE isNull(FullName,'Ed Jones') = 'Ed Jones'
Fixed: Select ... WHERE ((FullName = 'Ed Jones') OR (FullName IS NULL))

Bad: Select ... WHERE SUBSTRING(DealerName,4) = 'Ford'
Fixed: Select ... WHERE DealerName Like 'Ford%'

Bad: Select ... WHERE DateDiff(mm,OrderDate,GetDate()) >= 30
Fixed: Select ... WHERE OrderDate < DateAdd(mm,-30,GetDate()) 

@Mike Bailey 2012-06-22 12:26:21

Will including a function inside of GROUP BY cause a query to become non-sargable?

@BradC 2014-03-10 20:32:55

@MikeBantegui Just including a field in a GROUP BY won't necessarily make it non-sargeable, no. The right indexes will definitely help a GROUP BY query.

@Craig 2014-03-27 14:50:38

Some database engines (Oracle, PostgreSQL) support indexes on expressions, dontcha know?

@High Plains Grifter 2015-11-09 09:30:02

Would an even better version of WHERE ((FullName = 'Ed Jones') OR (FullName IS NULL)) be SELECT... FROM ... WHERE FullName = 'Ed Jones' UNION SELECT...FROM...WHERE FullName IS NULL? I was once told by an optimisation guy that using OR in the where clause can unsarg queries..?

@ad4s 2017-01-24 19:38:05

The UNION query would create temporary tables, which in fact may decrease overall performance for greater sets

@John Gibb 2018-01-12 15:23:13

@HighPlainsGrifter @ad4s I would recommend using explain analyze or similar to actually look at the query plans. Optimizers are pretty good at handling queries like that, so it's hard to say which will be faster in practice. There's also an element of non-determinism in there, as the query planner will give up trying to find a plan after a certain amount of time, and also base its decision off of table statistics. Always profile it to see!

@Devin Lamothe 2018-05-24 18:02:29

@HighPlainsGrifter you should use a UNION ALL on that query - union has an implicit distinct, which makes a query much more expensive than it needs to be when you have to mutually exclusive datasets

@CEGRD 2018-09-19 07:53:17

@BradC In MSSQL 2016, there is no execution plan difference between Select ... WHERE isNull(FullName,'Ed Jones') = 'Ed Jones' and Select ... WHERE ((FullName = 'Ed Jones') OR (FullName IS NULL)). They both use the index on FullName and do an index seek.

@beach 2009-04-28 20:05:19

Don't do this:

WHERE Field LIKE '%blah%'

That causes a table/index scan, because the LIKE value begins with a wildcard character.

Don't do this:

WHERE FUNCTION(Field) = 'BLAH'

That causes a table/index scan.

The database server will have to evaluate FUNCTION() against every row in the table and then compare it to 'BLAH'.

If possible, do it in reverse:

WHERE Field = INVERSE_FUNCTION('BLAH')

This will run INVERSE_FUNCTION() against the parameter once and will still allow use of the index.

@Adam Robinson 2009-04-28 20:16:12

Your suggestion with flipping the function would really only work when the function round-trips data (meaning that f(f(n)) = n).

@beach 2009-04-28 20:17:47

True. I considered adding INVERSE_FUNCTION but didn't want to be confusing. I'll change it.

@Dries Van Hansewijck 2009-04-28 21:14:37

In this answer I assume the database has sufficient covering indexes. There are enough questions about this topic.

A lot of the times the sargability of a query is determined by the tipping point of the related indexes. The tipping point defines the difference between seeking and scanning an index while joining one table or result set onto another. One seek is of course much faster than scanning a whole table, but when you have to seek a lot of rows, a scan could make more sense.

So among other things a SQL statement is more sargable when the optimizer expects the number of resulting rows of one table to be less than the tipping point of a possible index on the next table.

You can find a detailed post and example here.

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