index and data conversion

[sysdba]

Hi,

Table test(col1 varchar2(32) not null,
col2 number not null,
other_columns,,,)
index i_test on test(col1, col2)
Query select ... from test
where col1 = 'this is test'
and col2 = '12345';

I do not remember exactly whether I saw an article from
AskTom.com, saying this data conversion (col2) may cause
i_test index not being used.
When I used autotrace trace only, I find the index
is used.

Even changing index on test(col2), the index
is still used.

Can someone ********?

Thank you!

[slimdave]

the second column can be used through an "index skip-scan" access path in recent versions of Oracle -- also through a fast full index scan i suppose.

[sysdba]

Thank you, but I am using 8.1.7.

[slimdave]

i expect it might depend what was in the select clause -- the fast full scan is available in 8.1.7, and effectively treats the index as a skinny table.

[sysdba]

I found similiar question:
http://asktom.oracle.com/pls/ask/f?p...g4636346663651

Tom said:
=== BEGIN OF WHAT TOM SAID ========
If you have my book -- I answer your #3 question in "why isn't my index getting
used" in the chapter on indexes. column XXX is a varchar2 column. You are
comparing a string to a number. So

select * from t where xxx = 123

is REALLY

select * from t where to_number(xxx) = 123;

you are applying a function to the column XXX implicitly. That invalidates the
index usage. When you code:

select * from t where xxx = '123'

you are comparing a string to a string and lo and behold -- no such conversion
takes place.


ALWAYS COMPARE STRINGS TO STRINGS, DATES TO DATES, NUMBERS TO NUMBERS. do not
rely on implicit conversion -- ever!
=== END OF WHAT TOM SAID ========


How about my where clause:
A_NUMBER_COL = 'A_STRING_WHICH_CONTAIN_ONLY_DIGITS'
???

Is the above query is:
select ... from test where TO_CHAR(A_NUMBER_COL )='12345';
or
select ... from test where A_NUMBER_COL=TO_NUMBER('12345');

According my test of whether an index is used, it seems
to me that Oracle converts to:
select ... from test where A_NUMBER_COL=TO_NUMBER('12345');

Thanks

[sysdba]

I tested Tom's case (varchar2 col and where condition
is a_varchar2_col = 123) and Tom is always correct as usual.