oracle slides05 fp2005 ver 1.0

PL/SQL Performance tuning
Concurrency & Recovery
Data transfer utilities
Oracle Day 5
2
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Objectives

To understand PL/SQL performance tuning

To understand few tools like DBMS_TRACE used to tune the PL/SQL code.

To understand the locking mechanism and types of locking in Oracle.

To understand the read consistency model.

To understand the concurrency schemes followed by Oracle.

To understand different kinds of database failures and procedure to recover
from them.

To understand data transfer utilities
PL/SQL Performance tuning
4
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Tuning PL/SQL Applications
•
Usually developer efforts are concentrated on tuning the SQL’s used by the
PL/SQL program for performance improvement.
•
Being a procedural language, there can be situations where excessive CPU
usage arises out of the code in Pl/SQL even though there are no database
accesses.
•
By tuning the applications, one can make sure they continue to deliver the
required response time and throughput.
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Reasons for PL/SQL Performance Problems
•
Badly written SQL statements
•
Poor programming practices
•
Misuse of shared memory.
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Solution to Badly written SQL statements

•
Analyze the execution plans and performance using EXPLAIN PLAN
statement.
•
Rewrite the SQL statements.
•
For more info on SQL statements, please refer the RDBMS artifacts.
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Solutions to Poor programming practices

Here different areas of PL/SQL code are categorized and some of the basic
principles of optimizing are discussed:
•
DECLARE – what is required by the code
•
Hand crafting the Built-in Functions
•
Inefficient Conditional Control Statements
•
Check the LOOP Statements
•
Implicit Datatype Conversions
•
Inappropriate Declarations for Numeric Datatypes
•
Unnecessary NOT NULL Constraints

8
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DECLARE – what is required by the code
•
After the completion of the code, search for un-used variables if any. Remove
them from the code.
•
Defer the execution till it required.
DECLARE
l_chr_var1 VARCHAR2(15) :=
takes_five_miunte(… );
….
BEGIN
IF criteria1 THEN
use_the_defined_variable(l_chr_var1);
ELSE
say 90% of the cases follows this
Normal_code;
END IF;
END;
DECLARE
….
BEGIN
IF criteria1 THEN
DECALRE
l_chr_var1 VARCHAR2(15) :=
takes_five_miunte(… );

BEGIN
use_the_defined_variable(l_chr_var1);
END;
ELSE
Normal_code;
END IF;
END;
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Hand crafting the Built-in Functions
•
Built-in functions are more efficient.
•
Do not hand code one’s own versions of built-in functions such as REPLACE,
TRANSLATE, SUBSTR, INSTR, RPAD, and LTRIM. .
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Inefficient Conditional Control Statements
•
Most probable condition must be placed initially.
•
Now, consider the following AND expression:
IF credit_ok(cust_id) AND (loan < 5000) THEN

END IF;
The Boolean function credit_ok is always called.
•
However, if one switch the operands of AND as follows
IF (loan < 5000) AND credit_ok(cust_id) THEN

END IF;
The function is called only when the expression loan < 5000 is true
11
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Check the LOOP statements
•
Minimize the number of iterations inside loop. As soon as the code does the
required job EXIT the loop.
•
Loop within a loop – One common place where there is possibility of
unnecessary code execution.
•
Make sure that there should not be statements inside the loop that can be
executed outside the loop.
12
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Implicit Datatype Conversions
•

Avoiding implicit conversions can improve performance.
DECLARE
vNum NUMBER;
BEGIN
vNum:=vNum+15; converted
vNum:=vNum+15.0; not converted

END;
DECLARE
vChar CHAR(5);
BEGIN
vChar := 25; converted
vChar := '25'; not converted

END;
13
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Inappropriate Declarations for Numeric Datatypes

•
When one need to declare an integer variable, use the datatype
PLS_INTEGER, which is the most efficient numeric type.
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Unnecessary NOT NULL Constraints

•
Using the NOT NULL constraint incurs a performance cost.
PROCEDURE calc_m IS
m NUMBER NOT NULL := 0;
a NUMBER;
b NUMBER;
BEGIN

m := a + b;

END;
PROCEDURE calc_m IS
m NUMBER; no constraint
a NUMBER;
b NUMBER;
BEGIN

m := a + b;
IF m IS NULL THEN
enforce constraint programmatically

END IF;
END;
15
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Solution to Misuse of shared memory

•
Sizing the shared memory pool correctly.
•
Make sure it is large enough to hold all frequently used packages but not so
large that memory is wasted.
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Identifying PL/SQL Performance problems
•
There are few tools/API provided by PL/SQL to identify the PL/SQL
performance problems:
–
DBMS_PROFILER
–
DBMS_TRACE
•
One need DBA-SYS access to use these tools/API
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DBMS_PROFILER
•
The Profiler API is implemented as PL/SQL package DBMS_PROFILER.

•
Provides services for gathering and saving run-time statistics.
•
The information is stored in database tables, which one can query later.
•
For example, one can learn how much time was spent executing each PL/SQL
line and subprogram.
18
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DBMS_TRACE
•
The Trace API is implemented as PL/SQL package DBMS_TRACE.
•
Provides services for tracing execution by subprogram or exception.
•
One can see the order in which subprograms get executed.
•
In a typical session, follow the following steps:
–
Optionally, select specific subprograms for trace data collection.
–
Start by calling the procedure set_plsql_trace in package DBMS_TRACE.
–
Run the application to be traced.
–
Stop by calling the procedure clear_plsql_trace.
Concurrency in Oracle

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Locking mechanism
•
The smallest unit that can be locked in oracle is the row
•
For each SQL statement oracle acquires the appropriate lock
automatically.
•
Programmers cannot acquire locks at the row level explicitly
•
The smallest unit at which programmers can explicitly acquire locks is
the table
•
Locks are released when transaction commits or rolls back
•
Oracle does not acquire locks for reading
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Types of locks
•
When Oracle acquires locks at the row level, it automatically
acquires locks at the table level also
•

There are 5 different modes of locking at the table level
–
row share
–
row exclusive
–
share row exclusive
–
share
–
exclusive
22
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LOCK TABLE
LOCK TABLE
LOCK TABLE <tableName> IN <mode> MODE
23
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Locking Modes

EXCLUSIVE(X)

No other transaction can lock the table.


SHARE(S)

Allow queries but no updates

Other transactions can acquire S Lock on this table

Can allow SELECT FOR UPDATE & SELECT statements
24
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
ROW EXCLUSIVE(RX)

Allow concurrent access for queries, but prevent Share locking
also by other transactions

Permit Insert, Update, Delete or Select by other transactions/ or
lock rows in same table by other transactions

Prevent Share or Exclusive locking of the table by other
transactions
Locking Modes
25
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

ROW SHARE(=SHARE UPDATE)

Allow concurrent access, prevent Exclusive locking by other
transactions

SHARE ROW EXCLUSIVE(SRX)

Allow other users to look at table but prevent sharing and updates
by other transactions
Locking Modes