[Oracle] How to optimize queries - for Beginners

D.1 Optimizing Single-Table Queries

To improve the performance of a query that selects rows of a table based on a specific column value, create an index on that column. For example, the following query performs better if the NAME column of the EMP table has an index.

SELECT * 
FROM EMP 
WHERE NAME = 'Smith'; 

D.2 Optimizing Join Queries

The following can improve the performance of a join query (a query with more than one table reference in the FROM clause).

D.2.1 Create an Index on the Join Column(s) of the Inner Table

In the following example, the inner table of the join query is DEPT and the join column of DEPT is DEPT#. An index on DEPT.DEPT# improves the performance of the query. In this example, since DEPT# is the primary key of DEPT, an index is implicitly created for it. The optimizer will detect the presence of the index and decide to use DEPT as the inner table. In case there is also an index on EMP.WORKS_IN column the optimizer evaluates the cost of both orders of execution;DEPT followed by EMP (where EMP is the inner table) and EMP followed by DEPT (where DEPT is the inner table) and picks the least expensive execution plan.

SELECT e.SS#, e.NAME, d.BUDGET
FROM EMP e, DEPT d 
WHERE e.WORKS_IN = DEPT.DEPT# 
AND e.JOB_TITLE = 'Manager'; 

D.2.2 Bypassing the Query Optimizer

Normally optimizer picks the best execution plan, an optimal order of tables to be joined. In case the optimizer is not producing a good execution plan you can control the order of execution using the HINTS feature SQL. For more information see the Oracle9i Lite SQL Reference.
For example, if you want to select the name of each department along with the name of its manager, you can write the query in one of two ways. In the first example which follows, the hint /++ordered++/ says to do the join in the order the tables appear in the FROM clause with attempting to optimize the join order.

SELECT /++ordered++/ d.NAME, e.NAME
FROM DEPT d, EMP e
WHERE d.MGR = e.SS# 

or:

SELECT /++ordered++/ d.NAME, e.NAME 
FROM EMP e, DEPT d 
WHERE d.MGR = e.SS# 

Suppose that there are 10 departments and 1000 employees, and that the inner table in each query has an index on the join column. In the first query, the first table produces 10 qualifying rows (in this case, the whole table). In the second query, the first table produces 1000 qualifying rows. The first query will access the EMP table 10 times and scan the DEPT table once. The second query will scan the EMP table once but will access the DEPT table 1000 times. Therefore the first query will perform much better. As a rule of thumb, tables should be arranged from smallest effective number rows to largest effective number of rows. The effective row size of a table in a query is obtained by applying the logical conditions that are resolved entirely on that table.
In another example, consider a query to retrieve the social security numbers and names of employees in a given location, such as New York. According to the sample schema, the query would have three table references in the FROM clause. The three tables could be ordered in six different ways. Although the result is the same regardless of which order you choose, the performance could be quite different.
Suppose the effective row size of the LOCATION table is small, for example select count(*) from LOCATION where LOC_NAME = 'New York' is a small set. Based on the above rules, the LOCATION table should be the first table in the FROM clause. There should be an index on LOCATION.LOC_NAME. Since LOCATION must be joined with DEPT, DEPT should be the second table and there should be an index on the LOC column of DEPT. Similarly, the third table should be EMP and there should be an index on EMP#. You could write this query as:

SELECT /++ordered++/ e.SS#, e.NAME 
FROM LOCATION l, DEPT d, EMP e 
WHERE l.LOC_NAME = 'New York' AND 
l.LOC# = d.LOC AND 
d.DEPT# = e.WORKS_IN;

D.3 Optimizing with Order By and Group By Clauses

Various performance improvements have been made so that SELECT statements run faster and consume less memory cache. Group by and Order by clauses attempt to avoid sorting if a suitable index is available.

D.3.1 IN subquery conversion

Converts IN subquery to a join when the select list in the subquery is uniquely indexed.
For example, the following IN subquery statement is converted to its corresponding join statement. This assumes that c1 is the primary key of table t2:

SELECT c2 FROM t1 WHERE 
c2 IN (SELECT c1 FROM t2);

becomes:

SELECT c2 FROM t1, t2 WHERE t1.c2 = t2.c1;

D.3.2 ORDER BY optimization with no GROUP BY

This eliminates the sorting step for an ORDER BY clause in a select statement if ALL of the following conditions are met:

1. All ORDER BY columns are in ascending order or in descending order.
2. Only columns appear in the ORDER BY clause. That is, no expressions are used in the ORDER BY clause.
3. ORDER BY columns are a prefix of some base table index.
4. The cost of accessing by the index is less than sorting the result set.

D.3.3 GROUP BY optimization with no ORDER BY

This eliminates the sorting step for the grouping operation if GROUP BY columns are the prefix of some base table index.

D.3.4 ORDER BY optimization with GROUP BY

When ORDER BY columns are the prefix of GROUP BY columns, and all columns are sorted in either ascending or in descending order, the sorting step for the query result is eliminated. If GROUP BY columns are the prefix of a base table index, the sorting step in the grouping operation is also eliminated.

D.3.5 Cache subquery results

If the optimizer determines that the number of rows returned by a subquery is small and the query is non-correlated, then the query result will be cached in memory for better performance. Currently the number of rows is set at 2000. For example:

select * from t1 where 
t1.c1 = (select sum(salary) 
from t2 where t2.deptno = 100);

[Oracle] While creating a sequence, what does cache and nocache options mean?

One common question about sequences is:
Question: While creating a sequence, what does cache and nocache options mean? For example, you could create a sequence with a cache of 20 as follows:

CREATE SEQUENCE supplier_seq
MINVALUE 1
START WITH 1
INCREMENT BY 1
CACHE 20;

Or you could create the same sequence with the nocache option:

CREATE SEQUENCE supplier_seq
MINVALUE 1
START WITH 1
INCREMENT BY 1
NOCACHE;

Answer: With respect to a sequence, the cache option specifies how many sequence values will be stored in memory for faster access.
The downside of creating a sequence with a cache is that if a system failure occurs, all cached sequence values that have not be used, will be "lost". This results in a "gap" in the assigned sequence values. When the system comes back up, Oracle will cache new numbers from where it left off in the sequence, ignoring the so called "lost" sequence values.

Note: To recover the lost sequence values, you can always execute an ALTER SEQUENCE command to reset the counter to the correct value.

Nocache means that none of the sequence values are stored in memory. This option may sacrifice some performance, however, you should not encounter a gap in the assigned sequence values.

[Oracle] Enable / Disable all triggers in Schema

Disable Triggers

BEGIN
FOR i IN (SELECT trigger_name
FROM user_triggers) LOOP

EXECUTE IMMEDIATE 'ALTER TRIGGER ' || i.trigger_name || ' DISABLE';

END LOOP;
END;
/

Enable Triggers

BEGIN
FOR i IN (SELECT trigger_name
FROM user_triggers) LOOP

EXECUTE IMMEDIATE 'ALTER TRIGGER ' || i.trigger_name || ' ENABLE';

END LOOP;
END;
/

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[Oracle] How to see last sql statements executed

Recently we get into scenario where we need to see that which sql statements are being executed by activity upload.

So here is an easy way to do this:

select sql_text from v$sql where rownum<=100 and parsing_schema_name='NC_SDA_1_301';

Parsing Schema Name ; should be your schema name for which you need to see the sql statements executed on.

[Oracle] Enable / Disable all constraints in DB

Disable Constraints:

BEGIN

FOR c IN

(SELECT c.owner, c.table_name, c.constraint_name

FROM user_constraints c, user_tables t

WHERE c.table_name = t.table_name

AND c.status = 'ENABLED'

ORDER BY c.constraint_type DESC)

LOOP

dbms_utility.exec_ddl_statement('alter table ' || c.owner || '.' || c.table_name || ' disable constraint ' || c.constraint_name);

END LOOP;

END;

Enable Constraints:

BEGIN

FOR c IN

(SELECT c.owner, c.table_name, c.constraint_name

FROM user_constraints c, user_tables t

WHERE c.table_name = t.table_name

AND c.status = 'DISABLED'

ORDER BY c.constraint_type)

LOOP

dbms_utility.exec_ddl_statement('alter table ' || c.owner || '.' || c.table_name || ' enable constraint ' || c.constraint_name);

END LOOP;

END;

[T-SQL] Change SQL Server instance name

EXEC sp_dropserver 'Old Server Name'

EXEC sp_addserver 'New Server Name','local'

GO

Then restart the SQL service and confirm the new server name with @@SERVERNAME.

[Oracle] Update Data in Table trough Joins

create table source_table

(source_id    number primary key

,s_col1       number

,s_col2       number

,s_col3       number

);



create table target_table

(target_id    number primary key

,t_col1       number

,t_col2       number

,t_col3       number

);



insert into source_table(source_id,s_col1,s_col2,s_col3) values (101,111,1.01,-1);

insert into source_table(source_id,s_col1,s_col2,s_col3) values (102,222,2.02,-2);

insert into source_table(source_id,s_col1,s_col2,s_col3) values (103,333,3.03,-3);

insert into source_table(source_id,s_col1,s_col2,s_col3) values (104,444,4.04,-4);

insert into source_table(source_id,s_col1,s_col2,s_col3) values (105,555,5.05,-5);



insert into target_table(target_id,t_col1,t_col2,t_col3) values (101,null,null,null);

insert into target_table(target_id,t_col1,t_col2,t_col3) values (102,null,null,null);

insert into target_table(target_id,t_col1,t_col2,t_col3) values (103,null,null,null);

insert into target_table(target_id,t_col1,t_col2,t_col3) values (104,null,null,null);

insert into target_table(target_id,t_col1,t_col2,t_col3) values (105,null,null,null);

select * from source_table;
select * from target_table;

update
(
   select t_col1,t_col2,t_col3,s_col1,s_col2,s_col3
   from   target_table t
         ,source_table s
   where  t.target_id = s.source_id
)
set t_col1 = s_col1
   ,t_col2 = s_col2
   ,t_col3 = s_col3
;
select * from source_table;
select * from target_table;