Index-only tables

Oracle recognized that a table with an index built on every column did not require table rows to exist! In other words, Oracle recognized that by using a special table-access method called an index fast full scan, the index could be queried without actually touching the data itself.

Oracle codified this idea with its use of index-only table (IOT) structure. When using an IOT, Oracle does not create the actual table but instead keeps all of the required information inside the Oracle index.

At query time, the Oracle SQL optimizer recognizes that all of the values necessary to service the query exist within the index tree, at which time the Oracle cost-based optimizer has a choice of either reading through the index tree nodes to pull the information in sorted order or invoke an index fast full scan, which will read the table in the same fashion as a full table scan, using sequential prefetch (as defined by the db_file_multiblock_read_count parameter).

The multiblock read facility allows Oracle to very quickly scan index blocks in linear order, quickly reading every block within the index tablespace. Here is an example of the syntax to create an IOT.

CREATE TABLE emp_iot (

emp_id number,

ename varchar2(20),

sal number(9,2),

deptno number,

CONSTRAINT pk_emp_iot_index PRIMARY KEY (emp_id) )

ORGANIZATION index

TABLESPACE spc_demo_ts_01

PCTHRESHOLD 20 INCLUDING ename;

Information on IOT and pct_direct_access is available HERE.

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