RE: RE: Index-Organized Table experiences
- From: "Jacques Kilchoer" <Jacques.Kilchoer@xxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Wed, 28 Apr 2004 11:12:42 -0700
Oracle8i Concepts Release 2 (8.1.6) A76965-01=20
10 Schema Objects=20
Secondary Indexes on Index-Organized Tables=20
Secondary index support on index-organized tables provides efficient =
access to index-organized table using columns that are not the primary =
key nor a prefix of the primary key.=20
Oracle constructs secondary indexes on index-organized tables using =
logical row identifiers (logical rowids) that are based on the table's =
primary key. A logical rowid optionally includes a physical guess, which =
identifies the block location of the row. Oracle can use these guesses =
to probe directly into the leaf block of the index-organized table, =
bypassing the primary key search. Because rows in index-organized tables =
do not have permanent physical addresses, the guesses can become stale =
when rows are moved to new blocks.=20
For an ordinary table, access by a secondary index involves a scan of =
the secondary index and an additional I/O to fetch the data block =
containing the row. For index-organized tables, access by a secondary =
index varies, depending on the use and accuracy of physical guesses:=20
Without guesses, access involves two index scans: a secondary index scan =
followed by a scan of the primary key index.=20
With accurate guesses, access involves a secondary index scan and an =
additional I/O to fetch the data block containing the row.=20
With inaccurate guesses, access involves a secondary index scan and an =
I/O to fetch the wrong data block (as indicated by the guess), followed =
by a scan of the primary key index.
See Also: "Logical Rowids" =20
-----Original Message-----
Jared Still
Guess?
What does that mean?
An IOT is essentially a b*tree that is treated as a table.
What kind of guessing would be involved?
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