为什么不使用SQL Server索引？

Question

Most of my SQL queries have WHERE rec_id <> 'D'; as for example:

 select * from Table1 where Field1 = 'ABC' and rec_id <> 'D'

I added index on REC_ID . But when I run this query and look at the execution plan, the new index (REC_ID) is not used. The Execution plan shows Cost of 50% of nonClustered index Field1 and 50% RID Lookup (Heap) in Table1 .

Why the index REC_ID not used?

Answer 1

For this query:

select *
from Table1
where Field1 = 'ABC' and rec_id <> 'D';

The best index is table1(Field1, rec_id) .

However, your query may not be able to take advantage of an index. The goal of using an index for a where clause is to reduce the number of pages that need to be read. To understand the concept for non-clustered indexes on normal rows, you need some basic ideas:

• Records are stored on pages.
• Each page is 8,192 bytes (slightly fewer used for data) and can store some number of records.
• The entire page is loaded into memory to read a record.

Say a record is about 80 bytes and there are 100 records on each page. If 10% of the records have Field1 = 'ABC' , then there will be about ten on each page. That means that using the index would not (typically) save any page reads. If 1% of the records match, then there is about one on each page. The index still isn't helpful.

If only 0.01% of the records match (30 in your case), then only a fraction of the pages need to be read. This is the sweet spot for indexes, and where they are really helpful.

The number of matching records is called "selectivity". If the where clause is not very selective, then a non-clustered index will not be useful.

Sometimes, a clustered index can be helpful in this situation. However, clustered indexes may have more overhead for insert and certain update transactions. So, the choice of index needs to be based on the queries being processed and other ways that the table is used.

Answer 2

SQL Server uses many factors to decide which indices to use. It must have determined that using the index on Field1 would be more effective that using the index on rec_id - meaning that field1={value} defines a smaller set than rec_id <> {value} based on data dispersion, etc., so there are fewer records to compare against the other condition. Note that the actual value is usually irrelevant in determining which index to use.