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Redo AND Undo

陈举超

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2020-02-17

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2020-01-20 陈举超

Redo and Undo

but in my opinion the single most important feature of Oracle is one that first appeared in

version 6: the change vector, a mechanism for describing changes to data blocks, the heart

of redo and undo.

一 Basic Data Change 基础数据改变

One of the strangest features of an Oracle database is that it records your data twice. One copy of the data

exists in a set of data files which hold something that is nearly the latest, up-to-date version of your data

(although the newest version of some of the data will be in memory, waiting to be copied to disc); the other

copy of the data exists as a set of instructions—the redo log files—telling you how to re-create the content of

the data files from scratch.

1.1 The Approach 方法

2020-01-20 陈举超

Under the Oracle approach to data change, when you issue an instruction to change an item of data, Oracle

doesn’t just go to a data file (or the in-memory copy if the item happens to be buffered), find the item, and

change it. Instead, Oracle works through four critical steps to make the change happen.

Stripped to the bare minimum of detail, these are

1. Create a description of how to change the data item.

2. Create a description of how to re-create the original data item if needed.

3. Create a description of how to create the description of how to re-create the original data item.

4. Change the data item.

。

The tongue-twisting nature of the third step gives you some idea of how convoluted the mechanism is, but all

will become clear. With the substitution of a few technical labels in these steps, here’s another way of describing

the actions of changing a data block:

1. Create a redo change vector describing the change to the data block.

2. Create an undo record for insertion into an undo block in the undo tablespace.

3. Create a redo change vector describing the change to the undo block.

4. Change the data block.

The exact sequence of steps and the various technicalities around the edges vary depending on the version of

Oracle, the nature of the transaction, how much work has been done so far in the transaction, what the states

of the various database blocks were before you executed the instruction, whether or not you’re looking at the

first change of a transaction, and so on.

1.2 An Example 例：

I’m going to start with the simplest example of a data change, which you might expect to see as you updated a

single row in the middle of an OLTP transaction that had already updated a scattered set of rows. In fact, the

order of the steps in the historic (and most general) case is not the order I’ve listed in the preceding section.

The steps actually go in the order 3, 1, 2, 4, and the two redo change vectors are combined into a single redo

change record and copied into the redo log (buffer) before the undo block and data block are modified (in that

order). This means a slightly more accurate version of my list of actions would be:

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