Question

I’m running this query on an Oracle DB:

SELECT COUNT(1) FROM db.table WHERE columnA = 'VALUE' AND ROWNUM < 2

There is no index on columnA, and the table has many many thousands of lines (possibly millions). There’s about twenty values that should be returned, so it’s not a huge set being returned. However, because it triggers a full table scan it takes eons. How can I make it go faster?

Note: I’m not a DBA so I have limited access to the database and can’t implement restructuring, or adding indexes, or get rid of old data.

Answer 1

If you’re looking for the existence of a row, not the number of times it appears, then this would be more appropriate:

SELECT 1 
  FROM DB.TABLE
 WHERE ColumnA = 'VALUE'
   AND ROWNUM = 1

That will stop the query as fast as possible once a row’s been found; however, if you need it to go faster, that’s what indexes are for.

Test Case:

create table q8806566
( id        number not null,
  column_a  number not null,
  padding   char(256),  -- so all the rows aren't really short
  constraint pk_q8806566 primary key (id) 
    using index tablespace users
)
tablespace users;

insert into q8806566 -- 4 million rows
  (id, column_a, padding)
with generator as
(select --+ materialize
        rownum as rn from dba_objects 
  where rownum <= 2000)
select rownum as id, mod(rownum, 20) as column_a, 
       v1.rn as padding
  from generator v1
       cross join generator v2;

commit;

exec dbms_stats.gather_table_stats (ownname => user, tabname => 'q8806566');

The data for column_A is well distributed, and can be found in the first few blocks for all values, so this query runs well:

SELECT 1    
  FROM q8806566
 WHERE Column_A = 1 
   AND ROWNUM = 1;

Sub .1 sec execution time and low I/O – on the order of 4 I/Os. However, when looking for a value that’s NOT present, things change alarmingly:

SELECT 1    
  FROM q8806566
 WHERE Column_A = 20
   AND ROWNUM = 1;

20-40 seconds of execution time, and over 100,000 I/Os.

However, if we add the index:

create index q8806566_idx01 on q8806566 (column_a) tablespace users;
exec dbms_stats.gather_index_stats (ownname => user, indname => 'q8806566_idx01');

We get sub .1 second response time and single-digit I/Os from both queries.