Oracle Index Organized Table (IOT) |
Tables in RDBMS are heap-organized (unsorted), while the index organized tables are organized in a B-tree index structure and specified by a primary key. |
The B-tree (balanced tree) index structure is analogous to an inverted tree and the flow is from root-branch-leaf nodes. The index starts with a header node and leaf node with the header node consisting of a pointer to the leaf node and data stored in the leaf block. Each leaf block of the B-tree index structure stores both the key and nonkey columns. As the data volume increases, new leaf blocks are added. When header block gets filled up, new header blocks are added, which form branches. The B-tree index can be compressed to improve I/O, making IOT a good choice for OLTP systems. |
In the example below, when the length of a row exceeds 20% of the index block size (specified by PCTTHRESHOLD 20), the non-key columns are moved over to the overflow storage (specified by OVERFLOW clause). The including clause (specified by INCLUDING country_desc) forces the specified column(s) to be included in the key index leaf block. |
The PCTTHRESHOLD takes precedence over INCLUDING in the event of a conflict between PCTTHRESHOLD value and the INCLUDING clause. |
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