Doubt on High Water Mark

Oracle 9i

The high water to _TABLES s/n

The data dictionary keeps track of blocks that have been populated with lines. The high water is used as end marker during a full scan of the table.

High waters are stored in DBA_TABLES. BLOCKS

My doubt is

When the full Table Scan is performed

When the table contains fewer blocks DB_FILE_MULTI_BLOCK_READ_COUNT under high water, which can be read in a single I/O call, regardless of the fraction of the accessed tables or current index

Dount 1
which means according to my understanding, correct my if I'm wrong

That a table contains blocks and if the number of rows in the blocks is lower than statistics in DB_FILE_MULTI_BLOCK_READ the optimizer based on CSSTidy then performs a full table scan to retrieve the data in a single I/O transaction

Question 2
If a table TabA contains 3 blocks B1(A1,B1,C1), B2(A1,B2,C2), B3(A1,B3,C3).

Now if the data A1, B1, C1, A2, B2, C2 are deleted then the blocks B1, B2 are deleted or they will be empty

Question 3
When the last full scan of the table is performed on the Table in TabA, the high tide is set at top of B1.
(According to my understanding high water mark is set by the last full table on a table scan)

Then when data are removed i.e. edata in B1, B2 deleted then the high water mark is reset to B3 or is it still top of B1

If it's still high in B1 then
When the optimizer tries to determine what type of analysis to extract the table (A1, B3) data

So how it will determine if, to make the full Table Scan, i.e. If the tide is not reset then when he tried to pick up the A1 then block 1, 2 are empty, but if the tide is not reset then obviously the dat which can be read in a single I/O operation but the optimizer feels it's expensive because the high tide is not reset to B3

Thank you.

Not quite, but close. Perhaps a picture would help. For simplicity, I will assume that there is place for one line per block, but the basics are correct.

If you insert a new table 4 ranks, you'll have something like this:

                   H
                   W
-----------------------------------
 B1 | B2 | B3 | B4 |    |    |    |
-----------------------------------

where HW brand the leaves high-water, and the empty blocks on the right are attributed to the table (part of the original measure), but have never been used. If you delete the B1, B2 and B3, you'll have something like this:

                   H
                   W
-----------------------------------
    |    |    | B4 |    |    |    |
-----------------------------------

A full table scan, for some reason, all the blocks until the high water mark (i.e. the end of the B4) will read, because Oracle is unclear if there is indeed some lines in the block or not. He knows just that it might be because they are below the high water mark.

Now, if you insert B5 and B6 you'll get something like:

                   H
                   W
-----------------------------------
 B5 | B6 |    | B4 |    |    |    |
-----------------------------------

Since there is space below the high water mark. A full scan will still be read until the end of the B4. Now, if you add more than two lines B7 and B8, you get:

                        H
                        W
-----------------------------------
 B5 | B6 | B7 | B4 | B8 |    |    |
-----------------------------------

A row has been added below the high water mark, because there was space but the second was added in a new block and the high water mark was moved at the end last block used, and even if you remove all lines (removal of t) high water will remain in the B8 and a full scan reads all the 5 blocks.

John

Tags: Database

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for examplefind a watermark on a data file
Please specify.
Thank you

user8896122 wrote:

Hi all

I'm a little confused about that. Many links suggest that HWM is for Segments (data index etc.). But at the same time, I see a lot of discussion (on this forum as well) which suggest that the HWM is an attribute of the data file.

for examplefind a watermark on a data file

Please specify.

Thank you

When all else fails Read The Fine Manual (see LINK below)

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>
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>
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http://docs.Oracle.com/CD/B28359_01/server.111/b28318/logical.htm
>
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>

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>
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>
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Have you read this one as well, then?
```
A high water mark is the set of blocks that have at one pointed contained data.  You might have 1000 blocks allocated to a table but only 500 are under the HWM.  The blocks under the HWM are the blocks that will be read when the table is full scanned.
```
Ask Tom & quot; High-water & quot;

If that still is not make things clear, then please explain what is not clear to you.

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Insertion of the line in the table raises high tide and removing the high water mark where it is. If space below the high water mark is supposed to be exhausted while that above high tide as unused.
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Doubt on High Water Mark

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