Identifying a Poor performing SQL

Posted By Sagar Patil

Our preferred v$sqlarea query is to actually report on physical disk I/O per statement execution. Hit ratios are informative but sometimes misleading. Logical I/O is less relevant. If the statement executes 1,000,000 logical I/Os but still only takes less than one-tenth of a second, who cares? It is the total physical I/O that consumes nearly all the time and identifies the potentially incorrect SQL. For example:

SELECT sql_text, executions,
ROUND(disk_reads / executions, 2) reads_per_run,
disk_reads, buffer_gets,
ROUND((buffer_gets – disk_reads)/
buffer_gets, 2) hit_ratio,
FROM v$sqlarea
WHERE executions > 0
AND buffer_gets > 0
AND (buffer_gets – disk_reads) / buffer_gets < 0.80
ORDER by 3 desc ;

The previous two statements would have reported more enlightening results:
Executions Reads_Per_Run Disk_Reads Buffer_Gets Hit_Ratio Sql_Text
———- ————- ———- ———– ——— ————
2 3 6 19 0.68 SELECT …
2 1812.5 3625 178777 0.98 SELECT …
From this view of the v$sqlarea table, we can immediately isolate all statements that are performing high numbers of physical reads. These statements might not necessarily be inefficient or poorly written, but they are prime candidates for investigation and further tuning.

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