1. Use the unequal operator (<>, !=)
In the following case, even if the column dept_id has an index, the query still performs a full table scan.

select * from dept where staff_num <> 1000;

However, such a query is really needed in development. Is there a solution to the problem?

Have!

By using the or syntax instead of the inequality to query, you can use the index to avoid full table scan: the above statement is changed to the following, you can use the index.


select * from dept shere staff_num < 1000 or dept_id > 1000;

2, use is null or is not null

Using is null or is nuo null also limits the use of the index because the database does not define a null value. If there are many nulls in the indexed column, the index will not be used (unless the index is a bitmap index, the bitmap index will be explained in detail in a future blog post). Using null in the sql statement can cause a lot of trouble.

The solution to this problem is to define the column that needs to be indexed as non-null (not null) when building the table.

3, using the function

If you do not use a function-based index, the optimizer ignores the index when using a function on a column with an index in the where clause. The following query will not use the index:

select * from staff where trunc(birthdate) = '01-may-82';

But the function is applied to the condition, the index can be effective, and the above statement can be changed to the following statement, and the index can be searched.

select * from staff where birthdate < (to_date('01-may-82') + 0.9999);

4, compare data types that do not match

Comparing mismatched data types is also one of the performance issues that are difficult to find. In the following example, dept_id is a varchar2 type field with an index on this field, but the following statement performs a full table scan.

select * from dept where dept_id = 900198;

This is because oracle will automatically convert the where clause to to_number(dept_id)=900198, which is the case of 3, which limits the use of the index. Change the sql statement to the following form to use the index

select * from dept where dept_id = '900198';

5, use the like clause

When using the like clause to query, the data needs to be traversed to judge all the records, the index can not play a role, this situation should be avoided.


Looking at the introduction of foreign websites, the operators that affect the use of indexes are as follows:

"IS NULL", "<>", "!=", "!>", "!<", "NOT", "NOT EXISTS", "NOT IN", "NOT LIKE", and "LIKE '%500'"

In the actual test, it was found that "IN" was not included, and NOT IN was really slow.

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