1

While testing query:

select AccountID, SchemaID, Name, Description, CurrentVersion 
from [test].[dbo].[FormSchema] 
where SchemaID like '%[^a-zA-Z0-9-_]%'

After running execution plan it is showing clustered index scan with 100% cost. There is one clustered index on column schemaId.

My question is how can I make it index seek rather than scan?

  • 2
    The broad issue here is that your regex is not "sargable." Exactly what is and isn't sargable varies by platform, but as a heuristic, you could ask "could I quickly find someone in the phone book with this criteria?" The answer is "no," you'd need to look at every entry - in other words, an index scan. If your filter was SchemaID = 'Bob', you could skip to the Bs and flip a few pages left or right until you got to the Bobs - an index seek. – Jon of All Trades Mar 8 '17 at 22:19
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SchemaID like '%[^a-zA-Z0-9-_]%' is going to be problematic no matter what you do.

Having said that, if you added an index like this:

CREATE INDEX IX_FormSchema 
ON dbo.FormSchema(SchemaID) 
INCLUDE (AccountID, SchemaID, Name, Description, CurrentVersion);

And change the query to:

WHERE SchemaID like '[^a-zA-Z0-9-_]%'

you might get an index seek.

Having said that, an index scan is not inherently a bad thing. In fact, SQL Server is choosing to do an index scan because it believes that is the fastest way to get your results.

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