Performance Improvement - Part 2: Hard parse is not bad

Performance Improvement - Part 2: Hard parse is not bad

In previous post, we have seen the soft parsing is doing less operation comparing with hard parsing.

Please have a look on below post before read this one.
http://karthikeyanbaskaran.blogspot.com/2010/07/performance-improvement-part-1-parsing.html

The Hard parse includes the parse, optimize, generation of the plan for the query and soft parse skips optimize and generation of the plan. So we decided that hard parse is not good for application performance.

Some times soft parsing is having performance issue. The below scripts proves that.
SQL@ORA10G> create table t
2 as
3 select case when rownum = 1
4 then 1 else 99 end id, a.*
5 from all_objects a;

Table created.

SQL@ORA10G> create index t_idx on t(id);

Index created.


SQL@ORA10G> analyze table t compute statistics;

Table analyzed.


The WHERE ID=1 condition will return one record and WHERE ID=99 will return all of the rest (about 40,000 records). Also, the optimizer is very aware of this fact, we can definitely see different plans for different inputs, as we are passing the literals in the WHERE clause.

SQL@ORA10G> set autotrace traceonly explain
SQL@ORA10G> select * from t where id = 1;

Execution Plan
----------------------------------------------------------

0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=2 Card=1 Bytes=141)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'T' (TABLE) (Cost=2 Card=1 Bytes=141)
2 1 INDEX (RANGE SCAN) OF 'T_IDX' (INDEX) (Cost=1 Card=1)

SQL@ORA10G> select * from t where id = 99;

Execution Plan
----------------------------------------------------------

0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=137 Card=34323 Bytes=4839543)
1 0 TABLE ACCESS (FULL) OF 'T' (TABLE) (Cost=137 Card=34323 Bytes=4839543)





From Oracle9i Database Release 1 through Oracle Database 10g Release 2, Oracle Database will wait until the cursor is opened to do the actual optimization of the query—it will wait for the bind variable value to be supplied by the application before figuring out the right way to optimize the query. This is called bind variable peeking, when the optimizer first looks at the bind values and then optimizes the query.


In this case, however, depending on which inputs are used to first run the query, the database will either choose a full scan or an index range scan plus table access by index rowid. And in Oracle9i Database Release 1 through Oracle Database 10g Release 2, that is the plan that will be used to execute the SELECT * FROM t WHERE ID = :ID query, regardless of the subsequent bind values, until the query is hard-parsed and optimized again.

SQL@ORA10G> exec :id := 99

PL/SQL procedure successfully completed.

SQL@ORA10G> select * from t where id = :id;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=137 Card=20391 Bytes=1814799)

1 0 TABLE ACCESS (FULL) OF 'T' (TABLE) (Cost=137 Card=20391 Bytes=1814799)




So we started off with ID=99 as the bind, and the optimizer chose a full scan. Therefore, regardless of the bind value, the database will execute a full scan from now on. For example:
SQL@ORA10G> exec :id :=1

PL/SQL procedure successfully completed.

SQL@ORA10G> select * from t where id = :id;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=137 Card=20391 Bytes=1814799)

1 0 TABLE ACCESS (FULL) OF 'T' (TABLE) (Cost=137 Card=20391 Bytes=1814799)



The above result demonstrates that it is not going for the index range scan or table access by index rowid. In this case, the logical I/Os (Consistent gets) are more because of the FULL table scan instead of the index scan.

The result proves the “poorly performing query”.

Read More

Performance Improvement - Part 1: Parsing

Performance Improvement - Part 1: Parsing

In this article, we see how to improve the performance of the SQL queries. Before that we should know about the parsing and types of that.

Parsing:

This is the first step in the processing of any statement in Oracle. Parsing is the process of breaking the submitted statement down into its component parts. Determining what type of statement it is (whether Query, DML, DDL) and performing various checks on it.
The parsing process performs two main functions:

· Syntax Check
· Semantic Analysis

Syntax Check:

The syntax check performs the below operation.

Is the statement a valid one?
Does it make sense given the SQL grammar documented in the SQL Reference Manual? Does it follow all of the rules for SQL?

Semantic Analysis:
The semantic going beyond the syntax and it is performing the below operation.

Is the statement valid in light of the objects in the database (do the tables and columns referenced exist)?
Do you have access to the objects?
Are the proper privileges in place?
Are there ambiguities in the statement?
Soft parsing and hard parsing:
The next step in the parse operation is to see if the statement we are currently parsing has already in fact been processed by some other session. We may be in luck here If it has, we can skip the next two steps in the process, that is

1 .Optimization
2. Row source generation.

If we can skip these next two steps in the process, we have done what is known as a Soft Parse.


If we cannot, if we must do all of the steps, we are performing what is known as a Hard Parse. Hard parse includes the parse, optimize, generation of the plan for the query.
Example for Hard Parse:
The below scripts demonstrate the hard parsing.
SQL> ALTER SYSTEM FLUSH SHARED_POOL;

System altered.

SQL> create table t
(
id number(2),
Des varchar2(20)
);

Table created.

insert into t values(1,'AA');
insert into t values(2,'BB');
insert into t values(3,'CC');
insert into t values(4,'DD');
insert into t values(5,'EE');
insert into t values(6,'FF');


SQL> select * from t;

ID DES
--- --------
1 AA
2 BB
3 CC
4 DD
5 EE
6 FF

6 rows selected.


SQL> select SQl_TEXT,executions,hash_value,child_latch,child_address from v$sql where sql_text like 'select * from t where id %';

no rows selected

SQL> select * from t where id = 2;

ID DES
--- --------
2 BB

SQL> select * from t where id = 2;

ID DES
--- --------
2 BB

SQL> select * from t where id = 3;

ID DES
--- --------
3 CC

SQL> select SQl_TEXT,executions,hash_value,child_latch,child_address from v$sql where sql_text like 'select * from t where id %';




The above result is showing that, it is creating the different hash value for “where id = 2” and “where id =3 “. That means it is going for the hard parse for the each time when we change the condition.

Example for Soft parse:

The Execution column is showing 2 for the “where id =2 “. First time it has gone for the hard parse and second time it is gone for the soft parse.

How to make soft parse for “where id =2 “and “where id =3 “. We use the bind variable and it solves the issue.


SQL> variable id number
SQL> exec :id := 1;

PL/SQL procedure successfully completed.

SQL> select * from t where id = :id;

ID DES
--- --------
1 AA

SQL> exec :id :=2 ;

PL/SQL procedure successfully completed.

SQL> select * from t where id = :id;

ID DES
--- ------
2 BB

SQL> exec :id :=2 ;

PL/SQL procedure successfully completed.

SQL> select * from t where id = :id;

ID DES
--- -------
2 BB


SQL> select SQl_TEXT,executions,hash_value,child_latch,child_address from v$sql where sql_text like 'select * from t where id %';

It is not creating the different plan for each time when we use the bind variable. The hash value vale has created for the script “where id = :id “ and the value of the variable (:id) is changing each time. Refer the “Execution field, It is showing 3 for the different values.
Hard parsing is very CPU intensive. If our applications we want a very high performance then our queries to be Soft Parsed (to be able to skip the optimize/generate phases)

If we have to Hard Parse a large percentage of our queries, our system will function slowly and in some cases not at all.

Now we have a strong proof for the soft parsing and that improve the performance. But soft parsing is not always good and hard parsing is not always bad. We see that in the next Blog.

Read More

Reusable scripts - Part1

Find the letter occurrences in the statement:

The below query is finding the number of occurrence letter “a” in the string.

select length('Find a letter in a string') - length(replace('Find a letter in a string', 'a','')) as "Number of occurrence" from dual ;

Number of occurrence
--------------------
2


Remove the alphabetic letters in the given string:

declare
l_start number;
len number;
i number;
p_str varchar2(1000) := 'aa123yksjds45';
p_str1 varchar2(1000);
res varchar2(1000);
begin
i:=0;
len :=length(p_str);
WHILE i <= len
loop
len := ASCII(substr(p_str,i,1));
IF len > 47 AND len < 58 then
i:= i+1;
else
p_str := replace(p_str,substr(p_str,i,1),'');
END IF;
end loop;
dbms_output.put_line(p_str);
end;

Result:

12345

Divide the given comma separated words:

declare v_comma_position number;
v_len number;
p_comma_val varchar2(1000) := 'value1,value2,value3,value4,value5'; p_comma_tmp varchar2(1000);
v_result varchar2(1000);
begin p_comma_val := p_comma_val ',';
v_len :=length(p_comma_val);
WHILE v_len >= 0
loop
v_comma_position := instr(p_comma_val,',')+1;
p_comma_tmp := '';v_result := '';
p_comma_tmp := p_comma_val;
p_comma_val:='';
v_result := substr(p_comma_tmp,1,v_comma_position-2);
p_comma_val := substr(p_comma_tmp,v_comma_position,v_len);
v_len := v_len - v_comma_position;
dbms_output.put_line( v_result);
end loop;
end;

Result:

value1
value2
value3
value4
value5

Read More