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The library cache, being limited in size, discards old SQL statements when there s no more room for new SQL statements The only way you can use a parsed statement repeatedly for multiple executions is if a SQL statement is identical to the parsed statement Two SQL statements are identical if they use exactly the same code, including case and spaces The reason for this is that when Oracle compares a new statement to existing statements in the library cache, it uses simple string comparisons In addition, any bind variables used must be similar in data type and size Here are a couple of examples that show you how picky Oracle is when it comes to considering whether two SQL statements are identical.

In the following example, the statements aren t considered identical because of an extra space in the second statement: SELECT * FROM employees; SELECT * FROM employees; In the next example, the statements aren t considered identical because of the different case used for the table Employees in the second statement The two versions of employees are termed literals because they re literally different from each other SELECT * FROM employees; SELECT * FROM Employees; Let s say users in the database issue the following three SQL statements: SELECT * FROM persons WHERE person_id = 10 SELECT * FROM persons WHERE person_id = 999 SELECT * FROM persons WHERE person_id = 6666 Oracle uses a different execution plan for the preceding three statements, even though they seem to be identical in every respect, except for the value of person_id.

Synchronous blind interactions are the simplest interaction to implement and indeed the most common type used in software systems. They are typically accomplished using a simple method call. The caller makes a call to a function contained in the callee, and the caller blocks until the callee finishes. The call executes on the caller s thread and the callee returns control to the caller only when the command has been completely executed. If the command cannot be completed due to errors, the premature completion will still be regarded as a completion. When errors are expected, the interaction should arrange a way for the callee to notify the caller, perhaps by returning an error status code or throwing an exception. Figure 9-4 depicts a blind synchronous interaction.

Each of these statements has to be parsed and executed separately, as if they were entirely different Because all three are essentially the same, this is inherently inefficient As you can imagine, if hundreds of thousands of such statements are issued during the day, you re wasting database resources and the query performance will be slow Bind variables allow you to reuse SQL statements by making them identical, and thus eligible to share the same execution plan In our example, you can use a bind variable, which I ll call :var, to help Oracle view the three statements as identical, thus requiring a single execution instead of multiple ones The person_id values 10, 99, and 6666 are bound to the bind variable, :var.

Your replacement SQL statement using a bind variable, then, would be this: SELECT * FROM persons WHERE person_id = :var Using bind variables can dramatically increase query performance, and I explain in the section Using the CURSOR_SHARING (Literal Replacement) Parameter how you can force Oracle to use bind variables, even if an application doesn t use them..

The dictionary cache, as mentioned earlier, caches data dictionary information. This cache is much smaller than the library cache, and to increase or decrease it you modify the shared pool accordingly. If your library cache is satisfactorily configured, chances are that the dictionary cache is going to be fine too. You can get an idea about the efficiency of the dictionary cache by using the following query: SQL> SELECT (sum(gets - getmisses - fixed)) / SUM(gets) 2 "data dictionary hit ratio" from v$rowcache; data dictionary hit ratio ------------------------.936781093 SQL>

Figure 9-4. A synchronous blind interaction The notion of trust is important in this type of interaction. If the callee dies while executing the command, control may never return to the caller, and the whole system will appear to have died. A software system built exclusively using synchronous interactions will only be as reliable as the least reliable component. Components calling other components synchronously place a great deal of faith in the callees. For this reason, synchronous blind interactions are used most commonly with simple commands and when the probability of success is extremely high.

Usually, it s a good idea to shoot for a dictionary hit ratio as high as 95 to 99 percent, although Oracle itself sometimes seems to refer to a figure of 85 percent as being adequate. To increase the library cache ratio, you simply increase the shared pool size for the instance.