In practical applications, if the line sub-chain and the line moved Jingchang removed, then the block may not be reused in the free 绌洪棿, so in this case, we make use of space, it can be set to pctused high point during the operation mainly in the INSERT trigger line sub-chain operation. The line of migration caused mainly by the UPDATE operation.
Bank Link
Line links: Insert or Update cause, there is a different block of Row, row by different parts of different block address (ROWID) chain into a chain, can be set large DB_BLOCK_SIZE to reduce the occurrence of the link line, but can not be avoided (LOB) .
Bank Transfer: Update cause, as PCTFREE too small, ROWID unchanged, can be reduced by increasing the value of PCTFREE line of migration occurred.
Line sub-chain is not big enough DB_BLOCK_SIZE settings, so that the line for some big (like varchar2 (32666), blob, clob of length greater than the block size (eg 4K)) certainly can not save a block row, we need to keep separate line to a different block. in particular blob of this type, it is best to set up a separate table space to hold the actual data.
Contains a LONG / LONG RAW column, in this case, which is usually stored in the last Oracle maintain a LOB locator to allow quick access to the line outside the LOB. LOB locator, about 24 bytes, DISABLE STORAGE IN ROW save the line parameter to achieve outside the (scatter), regardless of data size of the row sub-chain is difficult to control (unless DB_BLOCK_SIZE by choosing large enough, can hold the line It is usually impossible). However, the line moved through the CREATE / ALTER TABLE command parameters PCTFREE and PCTUSED effective control.
Another line of links is the line occurred in the conduct of successful migration, such as UPDATE operation caused. When the update line to increase the size of the current free block is not enough space to lay down the line, then put the whole line to take out adequate space into a block (if not block up the necessary space for the entire line, line be divided into chain; if even if it is divided into chains still not enough space, then an error message "ORA-1650-55: Unable to extend).
Bank Transfer
If we want to update a row (update always allowed), and its line up, while the increase of the line is larger than the space required for block of free space. If we put a line in the extra space into another a block, while another line on the original block (which is the line of sub-link), so that block 100% of the original fill. This seems to only happen in a separate row, ORACLE will move this entire line to the other block can accommodate the space (this is the line of migration). This will remain in the original block in a line head, the first in line to retain the put the address of the actual line of block. The line links, but the application never know, use the SQL are no different, he just performance reasons. If the index to read through this line, the index will point to the first new block Not only is the use of two IO read about the index, plus a 1 IO to read this table to keep using PCTFREE free space per block, allowing OK still in the original block, not this kind, expansion.
If PCTFREE is 0, then all the free space will be used INSERT, UPDATE will line up to move to another block to fit the needs of expansion.
In the ideal case, set PCTFREE for the median size in line with the maximum values. For example, if the size of the median line is estimated at about 900 bytes, maximum line size of 1255 bytes, then set the PCTFREE value 10 (default) and 30 between the more reasonable.
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