BLOCK、EXTENT、SEGMENT、TABLESPACE、Data Dictionary

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<span style="font-family: 'Lucida Grande', Verdana, Arial, sans-serif; color: #111111; line-height: 15px;">Block
Block 可以分为 
  Header 
  Table Directory 
  Row Directory 
  Row Data
  Free Space
其中，block header、table directory、row directory 合起来称为 Overhead。

详细解释：

<div style="background-color: #f3f3f3; margin-top: 5px; margin-right: 20px; margin-bottom: 5px; margin-left: 20px; padding: 5px; border: 1px solid #cccccc;">Header (Common and Variable)
  The header contains general block information, such as the block address and the type of segment (for example, data or index).

Table Directory
  This portion of the data block contains information about the table having rows in this block.

Row Directory
  This portion of the data block contains information about the actual rows in the block (including addresses for each row piece in the row data area).this space is not reclaimed when the row is deleted.

Overhead
  The data block header, table directory, and row directory are referred to collectively as overhead. Some block overhead is fixed in size; the total block overhead size is variable. On average, the fixed and variable portions of data block overhead total 84 to 107 bytes.

Row Data
  This portion of the data block contains table or index data. Rows can span blocks.

Free Space
  Free space is allocated for insertion of new rows and for updates to rows that require additional space (for example, when a trailing null is updated to a nonnull value).