Index skip Scan – how?

With Oracle 9i, CBO is equipped with many more features, one of them is “Index skip scan” .This means even if you have a composite index on more than one column and you use the non-prefix column alone in your SQL, it may still use index.

I said “may” because CBO will calculate the cost of using the index and if it is more than that of full table scan, then it may not use index.

Index skip scan works differently from a normal index (range) scan.
A normal range scan works from top to bottom first and then move horizontal.
But a Skip scan includes several range scans in it. Since the query lacks the leading column it will rewrite the query into smaller queries and each doing a range scan.

Ex:
SQL> create table test (a number, b number, c number);

Table created.

SQL> create index test_i on test(a,b);

Index created.

SQL> begin
2 for i in 1 .. 100000
3 loop
4 insert into test values(mod(i, 5), i, 100);
5 end loop;
6 commit;
7 end;
8 /

PL/SQL procedure successfully completed.


SQL> exec dbms_stats.gather_table_stats(ownname => 'SAC', tabname => 'test', cascade => true);

PL/SQL procedure successfully completed.

SQL> set autotrace trace exp
SQL> select * from test where b=95267;

Execution Plan
----------------------------------------------------------
0
SELECT STATEMENT Optimizer=ALL_ROWS (Cost=22 Card=1 Bytes=10)


1 0
TABLE ACCESS (BY INDEX ROWID) OF 'TEST' (TABLE) (Cost=22 Card=1 Bytes=10)


2 1
INDEX (SKIP SCAN) OF 'TEST_I' (INDEX) (Cost=21 Card=1)


I above example, “select * from test where b=95267” was broken down to several small range scan queries. It was effectively equivalent to following

Select * from test where a=0 and b=95267
Union
Select * from test where a=1 and b=95267
Union
Select * from test where a=2 and b=95267
Union
Select * from test where a=3 and b=95267
Union
Select * from test where a=4 and b=95267;

In concrete, saying that skip scan is not as efficient as normal “single range scan” is correct. But yet saves some disk space and overhead of maintaining another index.