Essential Oracle Monitoring Queries for Performance Tuning

1. Wait Event Monitoring

select event,sum(decode(wait_time,0,0,1)) prev, sum(decode(wait_time,0,1,0)) curr,count(*) from v$session_wait group by event order by 4;

Lists each wait event with counts of previous and current wait times, helping identify spikes.

2. Rollback Segment Contention

select name,waits,gets,waits/gets ratio from v$rollstat a,v$rollname b where a.usn=b.usn;

Shows wait-to-get ratios for rollback segments to spot contention.

3. Tablespace I/O Ratio

select df.tablespace_name name,df.file_name "ile",f.phyrds pyr,f.phyblkrd pbr,f.phywrts pyw, f.phyblkwrt pbw from v$filestat f,dba_data_files df where f.file#=df.file_id;

Provides physical reads and writes per tablespace file.

4. File System I/O Ratio

select substr(a.file#,1,2) "#",substr(a.name,1,30) "name",a.status,a.bytes, b.phyrds,b.phywrts from v$datafile a,v$filestat b where a.file#=b.file#;

Shows I/O statistics for each data file.

5. Indexes Owned by a User

select user_indexes.table_name, user_indexes.index_name,uniqueness, column_name from user_ind_columns, user_indexes where user_ind_columns.index_name = user_indexes.index_name and user_ind_columns.table_name = user_indexes.table_name order by user_indexes.table_type, user_indexes.table_name, user_indexes.index_name, column_position;

Lists all indexes and their columns for a specific user.

6. SGA Logical vs Physical Reads (Buffer Hit Ratio)

select a.value + b.value "logical_reads", c.value "phys_reads", round(100 * ((a.value+b.value)-c.value) / (a.value+b.value)) "BUFFER HIT RATIO" from v$sysstat a, v$sysstat b, v$sysstat c where a.statistic# = 38 and b.statistic# = 39 and c.statistic# = 40;

Calculates the overall SGA buffer hit ratio.

7. Dictionary Cache Hit Ratio

select parameter, gets, Getmisses , getmisses/(gets+getmisses)*100 "miss ratio", (1-(sum(getmisses)/ (sum(gets)+sum(getmisses))))*100 "Hit ratio" from v$rowcache where gets+getmisses <>0 group by parameter, gets, getmisses;

Shows miss and hit percentages for the dictionary cache.

8. Library Cache Hit Ratio (Target < 1%)

select sum(pins) "Total Pins", sum(reloads) "Total Reloads", sum(reloads)/sum(pins) *100 libcache from v$librarycache;

select sum(pinhits-reloads)/sum(pins) "hit radio", sum(reloads)/sum(pins) "reload percent" from v$librarycache;

Evaluates library cache efficiency; a low reload percentage indicates good performance.

9. Object Size Summary

select count(name) num_instances ,type ,sum(source_size) source_size , sum(parsed_size) parsed_size ,sum(code_size) code_size ,sum(error_size) error_size, sum(source_size) +sum(parsed_size) +sum(code_size) +sum(error_size) size_required from dba_object_size group by type order by 2;

Aggregates size metrics for all database objects by type.

10. Redo Log Buffer Cache Hit Ratio (Target < 1%)

SELECT name, gets, misses, immediate_gets, immediate_misses, Decode(gets,0,0,misses/gets*100) ratio1, Decode(immediate_gets+immediate_misses,0,0, immediate_misses/(immediate_gets+immediate_misses)*100) ratio2 FROM v$latch WHERE name IN ('redo allocation', 'redo copy');

Measures redo allocation and copy latch efficiency.

11. Sort Area Ratio (Target < 0.10)

SELECT name, value FROM v$sysstat WHERE name IN ('sorts (memory)', 'sorts (disk)');

Helps decide whether to increase sort_area_size.

12. Active SQL Sessions

SELECT osuser, username, sql_text from v$session a, v$sqltext b where a.sql_address =b.address order by address, piece;

Shows which users are executing which SQL statements.

13. Dictionary Buffer Statistics

SELECT (SUM(PINS - RELOADS)) / SUM(PINS) "LIB CACHE" FROM V$LIBRARYCACHE;

SELECT (SUM(GETS - GETMISSES - USAGE - FIXED)) / SUM(GETS) "ROW CACHE" FROM V$ROWCACHE;

SELECT SUM(PINS) "EXECUTIONS", SUM(RELOADS) "CACHE MISSES WHILE EXECUTING" FROM V$LIBRARYCACHE;

Low ratios (< 1 %) indicate healthy cache performance.

14. Oracle Character Set

select * from sys.props$ where name='NLS_CHARACTERSET';

Retrieves the database character set.

15. MTS (Multi‑Threaded Server) Monitoring

select busy/(busy+idle) "shared servers busy" from v$dispatcher;

If the value exceeds 0.5, increase the dispatchers parameter.

select sum(wait)/sum(totalq) "ispatcher waits" from v$queue where type='dispatcher';

select count(*) from v$dispatcher;

select servers_highwater from v$mts;

When servers_highwater approaches mts_max_servers, raise the MTS limit.

16. Tablespace Fragmentation

select tablespace_name,count(tablespace_name) from dba_free_space group by tablespace_name having count(tablespace_name)>10;

Identifies tablespaces with many free fragments.

alter tablespace name coalesce;

alter table name deallocate unused;

Reclaims space.

create or replace view ts_blocks_v as select tablespace_name,block_id,bytes,blocks,'free space' segment_name from dba_free_space union all select tablespace_name,block_id,bytes,blocks,segment_name from dba_extents;

select * from ts_blocks_v;

select tablespace_name,sum(bytes),max(bytes),count(block_id) from dba_free_space group by tablespace_name;

Shows free space distribution.

SELECT segment_name table_name , COUNT(*) extents FROM dba_segments WHERE owner NOT IN ('SYS', 'SYSTEM') GROUP BY segment_name HAVING COUNT(*) = (SELECT MAX( COUNT(*) ) FROM dba_segments GROUP BY segment_name);

Finds tables with the highest fragmentation.

17. Table and Index Storage Checks

select segment_name,sum(bytes),count(*) ext_quan from dba_extents where tablespace_name='&tablespace_name' and segment_type='TABLE' group by tablespace_name,segment_name;

select segment_name,count(*) from dba_extents where segment_type='INDEX' and owner='&owner' group by segment_name;

Analyzes storage consumption for tables and indexes.

18. Sessions Consuming High CPU

Statistic #12 represents CPU used by a session.

select a.sid,spid,status,substr(a.program,1,40) prog,a.terminal,osuser,value/60/100 value from v$session a,v$process b,v$sesstat c where c.statistic#=12 and c.sid=a.sid and a.paddr=b.addr order by value;

Orders sessions by CPU usage.

19. Recently Executed SQL Statements

SELECT SQL_TEXT FROM V$SQL;

Retrieves the text of SQL statements that have been run.

20. Log Buffer Usage (Target < 1%)

select rbar.name,rbar.value,re.name,re.value, (rbar.value*100)/re.value||'%' "radio" from v$sysstat rbar,v$sysstat re where rbar.name='redo buffer allocation retries' and re.name='redo entries';

If the ratio exceeds 1 %, increase log_buffer size.