Lesson 8	Viewing the library cache statistics
Objective	Determine library cache misses and parse calls for SQL.

Viewing the Library Cache Statistics

Add Space to shared pool

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 more than one, you should consider adding to the shared_pool_size.

Library Cache Misses

Library cache misses occur during the compilation of SQL statements. The compilation of a SQL statement consists of two phases:

Parse 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
Execute phase: 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 re-parse and execute the statement.

Within the library cache, hit ratios can be determined for all dictionary objects that are loaded. If any of the hit ratios fall below 75 percent, you should add to the shared_pool_size. These include:

Table/procedures
Triggers
Indices
Package bodies
Clusters

Library cache activity

The table V$LIBRARYCACHE is the V$ table that keeps information about library cache activity.

Namespace	Indicates whether the measurement is for the SQL area, a table or procedure, a package body, or a trigger
Pins	Counts the number of times an item in the library cache is executed.
Reloads	Counts the number of times the parsed representation did not exist in the library cache, forcing Oracle to allocate the private SQL areas in order to parse and execute the statement

View the code below to see an example of library cache activity.

prompt         =========================
prompt         LIBRARY CACHE MISS RATIO
prompt         =========================
prompt (If > 1 then increase the shared_pool_size in init.ora)
prompt
column "LIBRARY CACHE MISS RATIO" format 99.9999
column "executions"    format 999,999,999
column "Cache misses while executing"    format 999,999,999
select sum(pins) "executions", 
    sum(reloads) "Cache misses while executing",
    (((sum(reloads)/sum(pins)))) "LIBRARY CACHE MISS RATIO"
from V$librarycache;
=========================
LIBRARY CACHE MISS RATIO
=========================
(If > 1 then increase the shared_pool_size in init.ora)
  executions Cache misses while executing LIBRARY CACHE MISS RATIO
------------ ---------------------------- ------------------------
650,059       967                         .0015
prompt
prompt         =========================
prompt          Library Cache Section
prompt         =========================
prompt hit ratio should be > 70, and pin ratio > 70 ...
prompt
column "reloads" format 999,999,999
select namespace, trunc(gethitratio * 100) "Hit ratio",
trunc(pinhitratio * 100) "pin hit ratio", reloads "reloads"
from V$librarycache;
=========================
Library Cache Section
=========================
hit ratio should be > 70, and pin ratio > 70 ...

Roll your cursor over the areas outlined below to read about the library cache section.

Terms 1) library cache pin and 2) library cache lock

library cache pin: This event manages library cache concurrency. Pinning an object causes the heaps to be loaded into memory. If a client wants to modify or examine the object, the client must acquire a pin after the lock.
library cache lock: This event controls the concurrency between clients of the library cache. It acquires a lock on the object handle so that either:
1. One client can prevent other clients from accessing the same object
2. The client can maintain a dependency for a long time which does not allow another client to change the object
This lock is also obtained to locate an object in the library cache.

If the library cache miss ratio is >1 then a larger shared_pool_size may be appropriate. If any of the library cache hit ratios or pin hit ratios is less than 90percent, then an increase in shared_pool_size may be appropriate.

Parse calls

High parse calls indicate those SQL statements that cannot be re-used and must be re-parsed at each execution. You will want to inspect these SQL statements to see if they can be made reentrant by adding host variables.

prompt 
********************************************************** 
prompt SQL High parse calls prompt
********************************************************** 
prompt select substr(sql_text,1,60), parse_calls, executions 
from
V$sqlarea where parse_calls > 300 and executions < 
2*parse_calls and executions > 1; 
SQL_TEXT
------------------------------------------------------------ 
PARSE_CALLS EXECUTIONS ----------- ------------ 
begin
dbms_output.disable; 
end; 
593 593 
begin 
dbms_output.enable(1000000); 
end; 
886 886 
begin 
dbms_output.get_lines(:lines, :numlines);
end; 
832 836

In the listing above we will see all SQL statements that have a high number of parse calls. In an ideal world, an SQL statement should be parsed once and executed numerous times. High parse counts and their solutions are addressed in the next module.
In the next lesson, we will examine the re-use in the library cache.

Viewing Library Cache Statistics - Exercise

Before you move on to the next lesson, click the Exercise link below to practice analyzing library cache statistics.
Viewing Library Cache Statistics - Exercise