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Don Burleson Blog 







 buffer pins Tips

Oracle Database Tips by Donald BurlesonMarch 14, 2015

Question:  What is a buffer pin, and what does the buffer is pined count mean in an AWR report?  I have over 83,000 buffer is pinned count per second and I don't know if I should be concerned.

Statistic                                     Total     per Second     per Trans
-------------------------------- ------------------ -------------- -------------
buffer is not pinned count               63,687,417       17,589.5         249.8
buffer is pinned count                  300,751,787       83,063.1       1,179.8

Answer:  A buffer pin is like a latch (cache buffer chains), and the buffer is pinned count defines a estimate of the efficiency of rare re-visits to the same data buffer block.  In plain English, a session may "pin" a handle to a data block to re-visit the data block at a later time without having to perform a second logical read. 

Oracle does not always pin a buffer block, and these types of non-latched reads are recorded as "consistent gets - examination" and "buffer is not pinned count" in the AWR report.  See my notes on consistent gets.

Oracle has several hidden parameters that define the buffer pin operations:

_buffer_handles_cached:  This defines the total number of concurrent pins allowed and the default is 5.

_db_handles:  This is he total number of "pins" that can be reserved by any database session.

These are hidden and undocumented parameters and should only be changed with the permission of MOSC.

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