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Printing USS Code 39 in Software Server process User process 1 5 4 System global area Database buffer cache

Server process User process 1 5 4 System global area Database buffer cache
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Datafiles
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Figure 8-4 The stages of execution of a SELECT
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8: DML and Concurrency
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In the figure, Step 1 is the transmission of the SELECT statement from the user process to the server process The server will search the database buffer cache to determine if the necessary blocks are already in memory, and if they are, proceed to Step 4 If they are not, Step 2 is to locate the blocks in the datafiles, and Step 3 is to copy them into the database buffer cache Step 4 transfers the data to the server process, where there may be some further processing before Step 5 returns the result of the query to the user process PART II
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Executing an UPDATE Statement
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For any DML operation, it is necessary to work on both data blocks and undo blocks, and also to generate redo: the A, C, and I of the ACID test require generation of undo; the D requires generation of redo EXAM TIP Undo is not the opposite of redo! Redo protects all block changes, no matter whether it is a change to a block of a table segment, an index segment, or an undo segment As far as redo is concerned, an undo segment is just another segment, and any changes to it must be made durable The first step in executing DML is the same as executing SELECT: the required blocks must be found in the database buffer cache, or copied into the database buffer cache from the datafiles The only change is that an empty (or expired) block of an undo segment is needed too From then on, things are a bit more complicated First, locks must be placed on any rows and associated index keys that are going to be affected by the operation This is covered later in this chapter Then the redo is generated: the server process writes to the log buffer the change vectors that are going to be applied to the data blocks This generation of redo is applied both to table block changes and to undo block changes: if a column of a row is to be updated, then the rowid and the new value of the column are written to the log buffer (which is the change that will be applied to the table block), and also the old value (which is the change that will be applied to the undo block) If the column is part of an index key, then the changes to be applied to the index are also written to the log buffer, together with a change to be applied to an undo block to protect the index change Having generated the redo, the update is carried out in the database buffer cache: the block of table data is updated with the new version of the changed column, and the old version of the changed column is written to the block of undo segment From this point until the update is committed, all queries from other sessions addressing the changed row will be redirected to the undo data Only the session that is doing the update will see the actual current version of the row in the table block The same principle applies to any associated index changes
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Executing INSERT and DELETE Statements
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Conceptually, INSERT and DELETE are managed in the same fashion as an UPDATE The first step is to locate the relevant blocks in the database buffer cache, or to copy them into it if they are not there
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Redo generation is exactly the same: all change vectors to be applied to data and undo blocks are first written out to the log buffer For an INSERT, the change vector to be applied to the table block (and possibly index blocks) is the bytes that make up the new row (and possibly the new index keys) The vector to be applied to the undo block is the rowid of the new row For a DELETE, the change vector to be written to the undo block is the entire row A crucial difference between INSERT and DELETE is in the amount of undo generated When a row is inserted, the only undo generated is writing out the new rowid to the undo block This is because to roll back an INSERT, the only information Oracle requires is the rowid, so that this statement can be constructed:
delete from table_name where rowid=rowid_of_the_new_row ;
Executing this statement will reverse the original change For a DELETE, the whole row (which might be several kilobytes) must be written to the undo block, so that the deletion can be rolled back if need be by constructing a statement that will insert the complete row back into the table
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