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Oracle Tips by Burleson Consulting
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Changing the
shared_pool_size parameter
We all know from Oracle8, that Oracle offers several queries for
determining when the Oracle shared pool is too small. The library
cache miss ratio tells the DBA whether or not to add space to the
shared pool, and it represents the ratio of the sum of library cache
reloads to the sum of pins.
In general, if the library cache ratio is over 1, you should
consider adding to the shared_pool_size. Library cache misses occur
during the parsing and preparation of the execution plans for SQL
statements. The compilation of a SQL statement consists of two
phases: the parse phase and the execute phase. When the time comes
to parse a SQL statement, Oracle first checks to see if the parsed
representation of the statement already exists in the library cache.
If not, Oracle will allocate a shared SQL area within the library
cache and then parse the SQL statement. At execution time, Oracle
checks to see if a parsed representation of the SQL statement
already exists in the library cache. If not, Oracle will reparse and
execute the statement.
The following STATSPACK script will compute the library cache miss
ratio. Note that the script sums all of the values for the
individual components within the library cache and provides an
instance-wide view of the health of the library cache.
rpt_lib_miss.sql
set lines 80;
set pages 999;
column mydate heading 'Yr. Mo Dy Hr.' format a16
column c1 heading "execs" format 9,999,999
column c2 heading "Cache Misses|While Executing" format 9,999,999
column c3 heading "Library Cache|Miss Ratio" format 999.99999
break on mydate skip 2;
select
to_char(snap_time,'yyyy-mm-dd HH24') mydate,
sum(new.pins-old.pins) c1,
sum(new.reloads-old.reloads) c2,
sum(new.reloads-old.reloads)/
sum(new.pins-old.pins) library_cache_miss_ratio
from
stats$librarycache old,
stats$librarycache new,
stats$snapshot sn
where
new.snap_id = sn.snap_id
and
old.snap_id = new.snap_id-1
and
old.namespace = new.namespace
group by
to_char(snap_time,'yyyy-mm-dd HH24')
;
Here is the output. The report above can easily be customized to
alert the DBA during times when there are excessive executions or
library cache misses.
Cache Misses
Yr. Mo Dy Hr. execs While Executing LIBRARY_CACHE_MISS_RATIO
---------------- ---------- --------------- ------------------------
2001-12-11 10 10,338 3 .00029
2001-12-12 10 182,477 134 .00073
2001-12-14 10 190,707 202 .00106
2001-12-16 10 2,803 11 .00392
Once this report identifies a time period where there may be a
problem, STATSPACK provides the ability to run detailed reports to
show the behavior of the objects within the library cache. In the
above example, we see a clear RAM shortage in the shared pool
between 10:00 AM and 11:00 AM each day. In this case we could
dynamically re-configure the shared pool with additional RAM memory
from the db_cache_size during this period.
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