Anti-called reverse key index index is not used to speed up data access, but in order to balance IO, designed to solve the thermal block
such data like this:
10000011000002 10000051000006
in the general index will appear on a leaf, if the department great demand data, i.e. thermal block, there will be a competition between a plurality of requests needs.
In order to avoid anti-competitive build key index reversed its data will
10000012000001 50000016000001
this index key can not be distributed on a leaf. IO achieved separation, each data in the index search will not match the path to solve the hot spot block competition.
Establish an anti-key index:
SQL> index i5 rebuild the ALTER Reverse;
Index altered.
Metadata search: SQL> select dbms_metadata.get_ddl ( 'INDEX', 'I5') from dual;
DBMS_METADATA.GET_DDL('INDEX','I5')
--------------------------------------------------------------------------------
CREATE INDEX "SCOTT"."I5" ON "SCOTT"."T1" (TRIM("ENAME"))
REVERSE
PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATI
STICS
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAX
EXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROU
PS 1 BUFFER_POOL DEFAULT)
TABLESPACE "USERS"
Anti-key index into the normal index:
SQL> alter index i5 rebuild noreverse;
Index altered.
Metadata search: SQL> select dbms_metadata.get_ddl ( 'INDEX', 'I5') from dual;
DBMS_METADATA.GET_DDL('INDEX','I5')
--------------------------------------------------------------------------------
CREATE INDEX "SCOTT"."I5" ON "SCOTT"."T1" (TRIM("ENAME"))
The INITRANS 2 255 10 MAXTRANS the PCTFREE the COMPUTE STATISTICS
STORAGE (the INITIAL MINE 65536 1048576 the NEXT
XTENTS. 1 the MAXEXTENTS 2147483645
PCTINCREASE the FREELISTS 0. 1. 1 FREELIST the GROUPS BUF
FER_POOL the DEFAULT)
TABLESPACE "the USERS"
has no REVERSE, description has been transferred to the normal general designated index.