Why a method to create a constraint would allow the index to be used, but not another.

Hi all

With the help of:

Oracle Database 11 g Enterprise Edition Release 11.2.0.4.0 - 64 bit Production

SQL Developer Version 4.0.2.15

I had to do the following query on a table.

Select distinct chdrnum, tranno

of ractpf_pgr;

I ran the setup of SQL in SQL Developer, who told me that he was doing a full table scan,

(It's what you get using SEPARATE!)

After some research, I discovered that if I put in question on columns not null check constraints

and then recreated the index based on these columns, I'd get a full index scan instead of a table scan.

Good news - off, I went and tried using the following method (I think it's call a constraint of out-of-line)

and created one for each colmn using snytax below.

ALTER table RACTPF_PGR add constraint CS_CHDRNUM_NN check (CHDRNUM is not null) validate;

Ran my SQL Tuning Advisor again - no joy - a full table scan that was happening.

Did some more research and recreated the constraints with the help of an online method

ALTER TABLE RACTPF_PGR CHANGE (CHDRNUM CONSTRAINT CS_CHDRNUM_NN NOT NULL);

My SQL Tuning Advisor is represented - and yes the index was used.


But what I want to know why a method would cause the index to use another does not


Thanks in advance.


The second is a NOT NULL constraint.

The a ('is not null') is technically not a NOT NULL constraint.

Give me a moment to start my database, and I will demonstrate.

Edit: Here's the demo:

SQL> create table x (a number null, b number null);                  

Table created.                                                       

SQL> alter table x add constraint a_nn check(a is not null) validate;

Table altered.                                                       

SQL> alter table x modify (b constraint b_nn not null);              

Table altered.                                                       

SQL> select column_name, nullable
  2  from user_tab_cols
  3  where table_name = 'X';                                         

COLUMN_NAME
----------------------------------------------------------------------

N
-
B
N                                                                    

A
Y

The index does not store registrations for the lines where all components of the index are null, then the optimizer needs to see that the columns are not nullable to be able to use an index scan.

Tags: Database

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