Availability and Optimization of Free Space in a Data Block(五)

最新推荐文章于 2025-12-20 13:10:13 发布

转载最新推荐文章于 2025-12-20 13:10:13 发布 · 125 阅读

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#数据库

本文探讨了通过DELETE和UPDATE语句释放数据库块空间的方法，并讨论了这些空间如何被后续的INSERT操作所利用。文章还介绍了Oracle数据库如何处理这些空闲空间，包括在何种条件下可以复用这些空间，以及如何在空间碎片化的情况下进行空间的合并。

Two types of statements can increase the free space of one or more data blocks:

DELETE statements, and UPDATE statements that update existing values to smaller

values.

The released space from these types of statements is available for subsequent

INSERT statements under the following conditions:

■ If the INSERT statement is in the same transaction and subsequent to the

statement that frees space, then the INSERT statement can use the space made

available.

■ If the INSERT statement is in a separate transaction from the statement that frees

space (perhaps being run by another user), then the INSERT statement can use the

space made available only after the other transaction commits and only if the space is needed.

Released space may or may not be contiguous with the main area of free space in a

data block. Oracle coalesces the free space of a data block only when (1) an INSERT or UPDATE statement attempts to use a block that contains enough free space to contain a new row piece, and (2) the free space is fragmented so the row piece cannot be

inserted in a contiguous section of the block.

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