Oracle Database - Consistent (Read get|Buffer Mode) (CR)


A consistent read get is a retrieval of a read-consistent version of a block from the buffer_cache and then may include read asides to UNDO (rollback segments).

A query will generally perform “consistent gets” and no current get, then the term logical I/O refer often to it.


If an uncommitted transaction has updated two rows in a block, and if a query in a separate session requests the block, then the database uses undo data to create a read-consistent version of this block (called a consistent read clone) that does not include the uncommitted updates. Typically, a query retrieves blocks in consistent mode.


Consistent gets statistic is a logical io (Buffer) statistic which give the number of times, a consistent read was requested for a block in the buffer_cache to get data from a data block.

Consistent reads may require read asides to the undo (rollback) information, and these reads to the undo will be counted as well.


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