SQL 'like' vs '=' performance
This question skirts around what I'm wondering, but the answers don't exactly address it.
It would seem that in general '=' is faster than 'like' when using wildcards. This appears to be the conventional wisdom. However, lets suppose I have a column containing a limited number of different fixed, hardcoded, varchar identifiers, and I want to select all rows matching one of them:
select * from table where value like 'abc%'
and
select * from table where value = 'abcdefghijklmn'
'Like' should only need to test the first three chars to find a match, whereas '=' must compare the entire string. In this case it would seem to me that 'like' would have an advantage, all other things being equal.
This is intended as a general, academic question, and so should not matter which DB, but it arose using SQL Server 2005.
Solution 1:
See https://web.archive.org/web/20150209022016/http://myitforum.com/cs2/blogs/jnelson/archive/2007/11/16/108354.aspx
Quote from there:
the rules for index usage with LIKE are loosely like this:
If your filter criteria uses equals = and the field is indexed, then most likely it will use an INDEX/CLUSTERED INDEX SEEK
If your filter criteria uses LIKE, with no wildcards (like if you had a parameter in a web report that COULD have a % but you instead use the full string), it is about as likely as #1 to use the index. The increased cost is almost nothing.
If your filter criteria uses LIKE, but with a wildcard at the beginning (as in Name0 LIKE '%UTER') it's much less likely to use the index, but it still may at least perform an INDEX SCAN on a full or partial range of the index.
HOWEVER, if your filter criteria uses LIKE, but starts with a STRING FIRST and has wildcards somewhere AFTER that (as in Name0 LIKE 'COMP%ER'), then SQL may just use an INDEX SEEK to quickly find rows that have the same first starting characters, and then look through those rows for an exact match.
(Also keep in mind, the SQL engine still might not use an index the way you're expecting, depending on what else is going on in your query and what tables you're joining to. The SQL engine reserves the right to rewrite your query a little to get the data in a way that it thinks is most efficient and that may include an INDEX SCAN instead of an INDEX SEEK)
Solution 2:
It's a measureable difference.
Run the following:
Create Table #TempTester (id int, col1 varchar(20), value varchar(20))
go
INSERT INTO #TempTester (id, col1, value)
VALUES
(1, 'this is #1', 'abcdefghij')
GO
INSERT INTO #TempTester (id, col1, value)
VALUES
(2, 'this is #2', 'foob'),
(3, 'this is #3', 'abdefghic'),
(4, 'this is #4', 'other'),
(5, 'this is #5', 'zyx'),
(6, 'this is #6', 'zyx'),
(7, 'this is #7', 'zyx'),
(8, 'this is #8', 'klm'),
(9, 'this is #9', 'klm'),
(10, 'this is #10', 'zyx')
GO 10000
CREATE CLUSTERED INDEX ixId ON #TempTester(id)CREATE CLUSTERED INDEX ixId ON #TempTester(id)
CREATE NONCLUSTERED INDEX ixTesting ON #TempTester(value)
Then:
SET SHOWPLAN_XML ON
Then:
SELECT * FROM #TempTester WHERE value LIKE 'abc%'
SELECT * FROM #TempTester WHERE value = 'abcdefghij'
The resulting execution plan shows you that the cost of the first operation, the LIKE comparison, is about 10 times more expensive than the = comparison.
If you can use an = comparison, please do so.
Solution 3:
You should also keep in mind that when using like, some sql flavors will ignore indexes, and that will kill performance. This is especially true if you don't use the "starts with" pattern like your example.
You should really look at the execution plan for the query and see what it's doing, guess as little as possible.
This being said, the "starts with" pattern can and is optimized in sql server. It will use the table index. EF 4.0 switched to like for StartsWith for this very reason.
Solution 4:
If value is unindexed, both result in a table-scan. The performance difference in this scenario will be negligible.
If value is indexed, as Daniel points out in his comment, the = will result in an index lookup which is O(log N) performance. The LIKE will (most likely - depending on how selective it is) result in a partial scan of the index >= 'abc' and < 'abd' which will require more effort than the =.
Note that I'm talking SQL Server here - not all DBMSs will be nice with LIKE.
Solution 5:
You are asking the wrong question. In databases is not the operator performance that matters, is always the SARGability of the expression, and the coverability of the overall query. Performance of the operator itself is largely irrelevant.
So, how do LIKE and = compare in terms of SARGability? LIKE, when used with an expression that does not start with a constant (eg. when used LIKE '%something') is by definition non-SARGabale. But does that make = or LIKE 'something%' SARGable? No. As with any question about SQL performance the answer does not lie with the query of the text, but with the schema deployed. These expression may be SARGable if an index exists to satisfy them.
So, truth be told, there are small differences between = and LIKE. But asking whether one operator or other operator is 'faster' in SQL is like asking 'What goes faster, a red car or a blue car?'. You should eb asking questions about the engine size and vechicle weight, not about the color... To approach questions about optimizing relational tables, the place to look is your indexes and your expressions in the WHERE clause (and other clauses, but it usually starts with the WHERE).