Reduced index rebuild the size of

Hi all

I think that this topic was raised by several times up to now... But I would like to know as I'm very curious.

An array of size 6 GB with index btree of 300 MB, the size very high CPU load in that server (I think because of I/O), after the reconstruction of this size of the index came up to 250 MB.

Now all my seniors ordered me to rebuild all indexes in the database that are huge in size, so that the i/o and CPU will be back to normal.

I had the argument with my elder for the reason why we need to rebuild the index?
Answer: As Oracle has access to this index it a lot of scan crashes a lot because of fragmentation. Oracle also has to scan all the blocks below the high water mark, where some blocks have fewer lines.

My doubt is: when index is fragmented (candidate means that after reconstruction, reduced its size) made Oracle scans unwanted blocks below let high water is wrong, because the INDEX and TABLE are two different concepts, INDEX maintains information on which all blocks, it would seek the required values. So Oracle will not scan the blocks unwanted in the INDEX.

Do may doubt is good or bad?

A few details:
The indicator is in DMT tablespace.
Oracle 8i version.
Sun solaris with 5.8 version, with 8 CPUS and 6 GB of RAM
None of the indices have a height more than two.

Plz HELP me in this concept... I want to prove my elders.

-Yasser

Yasserracdba,

People learn their theory IT more. The normal index is implemented as a B *-tree structure.
This means

                                                                  1 

                                             2        3                              5            6

                                      7       8    9   10                    11     12    13     14
                                                   

This means: block 1 contains information on block 2, 3, 5, 6 (I forgot 4, does not renumber)
Block 2 about 7 and 8
Block 7, etc are the leaves blocks and contain the actual keys.
The key consists of key values and a rowid.

The index is balanced. For a single scan will always tell you the same number of blocks, in this case 4
(3 blocks + block index 1 data).

IF YOU DON'T HAVE A CLUE, YOU WILL BE FORCED TO READ THE ENTIRE TABLE, THE TABLE SCAN COMPLETE THIS CALLED (YOU CAN SHOUT, I CAN SHOUT TOO)

TO ACCESS THE INDEX MEANS SELF-CONTAINED BED, FULL TABLE SCAN MEANS SCATTERED MULTI BLOCK READING (BLOCKS END UP NOT CONTIGUOUS CACHE)

FOR RANGE SCANS IO CAN BE REDUCED. TO LOOK UP A KEY INDIVIDUAL IO WILL NOT, I REPEAT N O T, REDUCE.

-----------------
Sybrand Bakker
Senior Oracle DBA

Experts: those who don't read the documentation

Published by: sybrand_b on May 28, 2009 11:04

Tags: Database

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