Trace Info

Question

When a trace a session we generally see the outputs in a tabular fashion for the PARSE,EXECUTE,FETCH phase like count,disk,elapsed time,query,current. According to the doc
QUERY: Total number of buffers retrived in consistent mode for each parse,execute,fetch phase.
CURRENT:Total number of buffers retrived in current mode for each parse,execute,fetch phase.
Could you please elaborate what that buffer retrived in consistent and current mode means.

Answer 1

" Current mode
A current mode get, also called a db block get, is a retrieval of a block as it currently appears in the buffer cache. For example, if an uncommitted transaction has updated two rows in a block, then a current mode get retrieves the block with these uncommitted rows. The database uses db block gets most frequently during modification statements, which must update only the current version of the block.
Consistent mode
A consistent read get is a retrieval of a read-consistent version of a block. This retrieval may use undo data. For example, 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."
http://docs.oracle.com/cd/E11882_01/server.112/e25789/memory.htm#CNCPT89169