How to Check Query

Question

Hi all,
How can you tell if a sql query is poorly written or not ( optimized or not)?
Is it by using Explain Plan ? There should be no full table scan it the report? and cost less than < what number?
Thanks a lot

Answer 1

I will just in continue to Sybrand i.e. if you get the output of below query :
SELECT * FROM
(SELECT
sql_fulltext,
sql_id,
child_number,
disk_reads,
executions,
first_load_time,
last_load_time
FROM v$sql
ORDER BY elapsed_time DESC)
WHERE ROWNUM < 10
This finds the top SQL statements that are currently stored in the SQL cache, you can take them "Poor SQLs".
You could find disk intensive full table scans with something like this:
SELECT Disk_Reads DiskReads, Executions, SQL_ID, SQL_Text SQLText,
SQL_FullText SQLFullText
FROM
SELECT Disk_Reads, Executions, SQL_ID, LTRIM(SQL_Text) SQL_Text,
SQL_FullText, Operation, Options,
Row_Number() OVER
(Partition By sql_text ORDER BY Disk_Reads * Executions DESC)
KeepHighSQL
FROM
SELECT Avg(Disk_Reads) OVER (Partition By sql_text) Disk_Reads,
Max(Executions) OVER (Partition By sql_text) Executions,
t.SQL_ID, sql_text, sql_fulltext, p.operation,p.options
FROM v$sql t, v$sql_plan p
WHERE t.hash_value=p.hash_value AND p.operation='TABLE ACCESS'
AND p.options='FULL' AND p.object_owner NOT IN ('SYS','SYSTEM')
AND t.Executions > 1
ORDER BY DISK_READS * EXECUTIONS DESC
WHERE KeepHighSQL = 1
AND rownum <=5;
Source:http://stackoverflow.com/questions/316812/top-5-time-consuming-sql-queries-in-oracle
Regards
Girish Sharma