Quoted from OraFAQ Mail list:
Q: What is db block gets and consistent gets?
How can I reduce consistent gets?
Ask Tom says each consistent gets is latch, how it could be?
A: A 'db block get' is a current mode get. That is, it's the most up-to-date
copy of the data in that block, as it is right now, or currently. There
can only be one current copy of a block in the buffer cache at any time.
Db block gets generally are used when DML changes data in the database.
In that case, row-level locks are implicitly taken on the updated rows.
There is also at least one well-known case where a select statement does
a db block get, and does not take a lock. That is, when it does a full
table scan or fast full index scan, Oracle will read the segment header
in current mode (multiple times, the number varies based on Oracle version).
A 'consistent get' is when Oracle gets the data in a block which is consistent
with a given point in time, or SCN. The consistent get is at the heart of
Oracle's read consistency mechanism. When blocks are fetched in order to
satisfy a query result set, they are fetched in consistent mode. If no
block in the buffer cache is consistent to the correct point in time, Oracle
will (attempt to) reconstruct that block using the information in the rollback
segments. If it fails to do so, that's when a query errors out with the
much dreaded, much feared, and much misunderstood ORA-1555 "snapshot too old".
As to latching, and how it relates, well, consider that the block buffers
are in the SGA, which is shared memory. To avoid corruption, latches are
used to serialize access to many linked lists and data structures that point
to the buffers as well as the buffers themselves. It is safe to say that
each consistent get introduces serialization to the system, and by tuning
SQL to use more efficient access paths, you can get the same answer to the
same query but do less consistent gets. This not only consumes less CPU,
it also can significantly reduce latching which reduces serialization and
makes your system more scalable.
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