view columns in Oracle
The X$ fixed views
provide a wealth of internal information about the behavior of the data
buffer caches, and the addition of the following columns to X$BH adds
additional data. These
include the LRU_FLAG, TCH, and TIM columns.
Let’s take a closer
look at each of these undocumented columns and see if we can infer their
purpose inside Oracle.
The LRU_FLAG column of
X$BH contains information about LRU state of a block in the buffer cache,
but this column does not reveal much useful information.
The TIM column appears
to be related to the hidden parameter _DB_AGING_TOUCH_TIME, and this
metric appears to be associated with the new mid-point data buffer
insertion algorithm that was introduced in Oracle8i. On the surface, it appears that the duration between RAM
touches in the data buffer is tangentially related to Oracle’s internal
movement of the data block to the MRU movement of the buffer.
It appears that the TCH
column measures the number of times that a specific data block has been
touched by an SQL query. As
we know from Oracle Internals, the Oracle9i data buffers are designed to segregate
data into hot regions and cold regions, and the internal processes are
used to segregate data blocks according to the frequency that they have
The TCH column appears
to be related to the hidden Oracle9i parameter _DB_AGING_HOT_CRITERIA and
this present some very useful insights.
As we know, one of the
challenges of the Oracle DBA is to identify candidates for each of the
eight data pools within Oracle. We
have the DEFAULT pool, the KEEP pool, the RECYCLE pool, as well as
instantiated pools for each supported blocksize on the Oracle server.
The TCH column can be used to infer the relative popularity of data
blocks from each table or index, and this data can be used to make
intelligent decisions about the segregation of these database objects into
their separate data buffers.
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