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Row Format and Size (156)

原创 IT综合 作者:tsinglee 时间:2007-11-07 11:30:32 0 删除 编辑

Oracle stores each row of a database table containing data for less than 256 columns as
one or more row pieces. If an entire row can be inserted into a single data block, then
Oracle stores the row as one row piece. However, if all of a row’s data cannot be
inserted into a single data block or if an update to an existing row causes the row to
outgrow its data block, then Oracle stores the row using multiple row pieces. A data
block usually contains only one row piece for each row. When Oracle must store a row
in more than one row piece, it is chained across multiple blocks.

When a table has more than 255 columns, rows that have data after the 255th column
are likely to be chained within the same block. This is called intra-block chaining. A
chained row’s pieces are chained together using the rowids of the pieces. With
intra-block chaining, users receive all the data in the same block. If the row fits in the
block, users do not see an effect in I/O performance, because no extra I/O operation is
required to retrieve the rest of the row.

1. Oacle存储小于256列的数据行时 , 如果一整行能被存储在一个数据块中 , 那么将作为一个行片段来保存
2. 当一个数据块不能存储一个行片段时 , 在多个块之间产生链接
3. 当表列大于255列时 , 后面的列可能在相同的块内连接


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