Oracle: Fastest Way to Extract Filename Extension Using SQL or PL/SQL

Question

I need to get the extensions of filenames. Extensions could be any length (not just 3) and they could also be non-existent, in which case I need null returned. I know I could easily write a PL/SQL function that does this then just call that function in the query but I was hoping that I could somehow do it all inline. And I don't really care how long the solution is, what I need is the fastest solution. Speed matters because this will end up being ran against a very large table. This is what I have so far...

/*
The same method is being used in all 5 examples.
It works for all of them except the first one.
The first one I need to return null
*/

SELECT substr(filename,instr(filename,'.',-1)+1,length(filename)-instr(filename,'.',-1))
  FROM (select 'no_extension_should_return_null' filename from dual);
--returns: no_extension_should_return_null

SELECT substr(filename,instr(filename,'.',-1)+1,length(filename)-instr(filename,'.',-1))
  FROM (select 'another.test.1' filename from dual);
--returns: 1

SELECT substr(filename,instr(filename,'.',-1)+1,length(filename)-instr(filename,'.',-1))
  FROM (select 'another.test.doc' filename from dual);
--returns: doc

SELECT substr(filename,instr(filename,'.',-1)+1,length(filename)-instr(filename,'.',-1))
  FROM (select 'another.test.docx' filename from dual);
--returns: docx

SELECT substr(filename,instr(filename,'.',-1)+1,length(filename)-instr(filename,'.',-1))
  FROM (select 'another.test.stupidlong' filename from dual);
--returns: stupidlong

So is there a fast way to accomplish this inline or should I just write this in a PL/SQL function?

This is what I'm working with...

select * from v$version;
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE    11.2.0.2.0  Production
TNS for 64-bit Windows: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production

UPDATE I'm moving this code into a function and will setup a test to call it a million times to see if the function slows it down, I'm thinking it won't make an impact since it's just string manipulation.

UPDATE Thanks for the answers so far. I ended up making a PL/SQL function that does what I need...

create or replace function extrip(filename varchar2) return varchar2 as
begin
    if ( instr(filename,'.',-1) = 0 ) then
        return null;
    end if;

    return substr(filename,instr(filename,'.',-1)+1,length(filename)-instr(filename,'.',-1));
end;

I then ran two tests against a table with 2 million rows. When I viewed the explain plan for both they were 100% IDENTICAL. How could that be?

select regexp_substr(filename, '\.[^\.]*$') ext from testTable;

select extrip(filename) ext from testTable;

UPDATE I added a order by ext to both of those then reran the tests and there was a difference. The regexp took 9sec and the function took 17sec. I guess without the order by TOAD was just retrning the first X number of recs. So @Brian McGinity was right. I still need the regexp method to NOT return the dot "." though.

Answer 1

It will run fastest when done 100% sql, as you have.

The substr/instr are native compiled functions in oracle.

If you put this in a plsql function it will run slower due to context switching between sql and plsql:

This is slower due to context switching:

select extrip( filename ) from million_row_table 

What you have is faster.

Update:

try this:

select s,
       substr(s,   nullif( instr(s,'.', -1) +1, 1) )
from ( 
     select 'no_extension_should_return_null' s from dual union
     select 'another.test.1'                    from dual union
     select 'another.test.doc'                  from dual union
     select 'another.test.docx'                 from dual union
     select 'another.test.stupidlng'            from dual 
     )

Answer 2

You need to use regular expressions.

Try

select regexp_substr(filename, '\.[^\.]*$')
from
    (select 'no_extension_should_return_null' filename from dual);

I don't have an Oracle database to test this on but this should be pretty close.

Check the Oracle docs on regexp_substr and Using regular expressions in Oracle database for more info.

Update

To drop the period from the file extension:

select substr(regexp_substr(filename, '\.[^\.]*$'), 2)
from
    (select 'abc.def' filename from dual);

Answer 3

SELECT NULLIF(substr(filename,instr(filename,'.',-1)+1,length(filename)-instr(filename,'.',-1)) from (select 'no_extension_should_return_null' filename from dual) t1, SELECT filename from t1);

对不起没有oracle测试它，我相信你明白了。

Answer 4

是的根据我的理解你可以使用DECODE函数和查询如下：

SELECT substr(filename,instr(filename,'.',-1)+1,length(filename)- DECODE(INSTR(filename,'.',-1),0,LENGTH(filename),INSTR(filename,'.',-1))) from (select 'no_extension_should_return_null' filename from dual);

Answer 5

Perhaps the simplest would be to use

regexp_substr(filename, '[^\\.]*$')

It works on filenames with multiple periods and returns no period.

---

For filenames without extension next could be used

select case when filename like '%.%' then regexp_substr(filename, '[^.]*$') end EXT from dual