What is the right time to use the index with force?

I have an EMPLOYEE table.  I join with the ROLE, and it has only about 200 distinct values on column EMPLOYEE. EMPLOYEE_TYPE_ID.

Select / * + INDEX (an i_employee_type_id) * / b.SID as EMP_NAME, b.role_cd in the ROLE

Of

EMPLOYEE,

B ROLE

where

a.EMPLOYEE_TYPE_ID = b.EMPLOYEE_TYPE_ID

AND a.EFFECTIVE_END_TS > = systimestamp;

Is it a good idea to use the index?  .. Or let the full table scan.

SQL > select distinct EMPLOYEE_TYPE_ID of the EMPLOYEE;

238 selected lines.

If you don't know it will help not to use.

Personally, I found several SQLs with index finger tips (written by programmers thinking index access is ALWAYS GOOD) end up harming performance. Oracle made a reading diluvium full table scan, single index reads as follows, according to the % of the returned array it is actually faster to do a full table scan. The optimizer did a great job to determine this.

If you think there are cardinality estimation problems, and the optimizer expects a large number of lines when there are actually few being returned, in THIS case a suspicion was justified. Even so, in this case, I prefer that oracle manages the flag itself.

You can do this by running the SQL tuning advisor. In fact, it will trigger the optimizer checks that the estimates are turned off and it will create a profile for you. (A profile is actually a stored set of advice that set the execution plan for you). If at the point where the underlying data changes significantly and profile ends up hurting performance, you do not need to touch the code like you would with manually added notes, you can just disable or delete the profile and let the optimizer re - analyze the statement.

Concerning

EDIT: In case of small tables, it is preferable to just cache the whole table in the POOL to KEEP and let oracle scan if necessary.

Edit2: With the notable exception of index fast full scans, which are diluvium index readings. But they are only relevant when the request is quite satisfied by the index and has no need to visit the table.

Tags: Database

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