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Home -> Community -> Mailing Lists -> Oracle-L -> RE: PCTTHRESHOLD in IOT
Hi Arvind,
This comes from the Oracle 8i Concepts manual, chapter 10 on Schema objects.
Sam.
Index-Organized Tables with Row Overflow Area B-tree index entries are usually quite small since they only consist of the pair <key, ROWID>. In index-organized tables, however, the B-tree index entries can be very large since they consist of the pair <key, non_key_column_values>. If the index entry gets very large, then the leaf nodes may end up storing one row or row-piece, thereby destroying the dense clustering property of the B-tree index.
Oracle provides an OVERFLOW clause to handle this problem. You can specify an overflow tablespace as well as a threshold value. The threshold is specified as a percentage of the block size (PCTTHRESHOLD).
If the row size is greater than the specified threshold value, then the non-key column values for the row that exceeds the threshold are stored in the specified overflow tablespace. In such a case the index entry contains a <key, rowhead> pair, where the rowhead contains the beginning portion of the rest of the columns. It is like a regular row-piece, except it points to an overflow row-piece that contains the remaining column values.
See Also:
Oracle8i Administrator's Guide for examples of using the OVERFLOW clause
-----Original Message-----
From: Arvind Aggarwal [mailto:oradba9_at_yahoo.com]
Sent: January 25, 2001 12:36 PM
To: Multiple recipients of list ORACLE-L
Subject: PCTTHRESHOLD in IOT
Hi ALL,
I have a doubt for PCTTHRESHOLD parameter in IOT. When we say that this IOT has 20 PCTTHRESHOLD, it means that it forces some rows into the overflow segment but on which basis Is this applicable to all rows in the block or some rows????. When we are saying 20 PCTTHRESHOLD what exactly that means , 20 is refering to 20% of block size????.
when I wan't that the same set of columns has to be in the index and the rest of the columns after that in the overflow segment, I have to specify upto which column i wan't to keep in the index with INCLUDING. What happened if the corresponding rowsize exceeds and unable to put the last column specified in the INCLUDING columns, where will that column go, in the index segment or in the overflow segment.
Thanks in advance,
Arvind
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