Queries can lock tables:
SELECT... FOR UPDATE;
I find this is a common cause of blocking and is easy to overlook as you might
miss the FOR UPDATE.
Table DDL can also lock the resource by dropping a TM lock on there.
Neil.
sent from my phone
On 29 Oct 2018, at 03:36, Mladen Gogala
<gogala.mladen@xxxxxxxxx<mailto:gogala.mladen@xxxxxxxxx>> wrote:
Hi Eriovaldo!
Queries do not lock tables, unless the queries are distributed, as in "query
over a database link". For everything else, queries to not lock tables.
Second, the fact that the sessions are idle is irrelevant. If I execute "LOCK
TABLE SCOTT.EMP IN EXCLUSIVE MODE" and go to lunch, my session will be idle. It
will also keep the lock until the transaction ends. Locks are caused by 5
statements: INSERT, UPDATE, DELETE, TRUNCATE and LOCK. For the purpose of this
consideration, we will consider MERGE to be an update & insert combination.
So, if you have a SID, you should be able to find out what objects does the
session have locked and in what mode. Sometimes, that can be a load of fun.
While I was a DBA, I worked with a developer who learned that /*+ APPEND */
hint for inserts "makes things go faster". So she has put the /*+ APPEND */
hint to each and every insert statement in her multi-user web application. For
some inexplicable reason, things did not go faster, but I did have some fun.
* Idle sessions can hold locks, for a very long time. That was the primary
reason for inventing resource limits.
* Queries, except in very rare cases, DO NOT LOCK anything.
* V$LOCK will tell what objects are locked and in what mode. Unless you
are using 18.3 or newer, you cannot kill a statement (actually, not true: you
can kill the current statement by using kill -URG, but very few people know
that and even fewer do that).
* Once you figure out that an idle session is blocking you by holding a
lock, you kill it without further ado and ask the application developer to fix
the bug. There is absolutely no reason whatsoever for an idle session to hold
locks. Locks are means for preserving consistency. You want to prevent someone
else from modifying a critical resource until your transaction is finished. You
don't hold locks on resources until an upgrade to a new Oracle version takes
you apart. TRANSACTION != MARRIAGE.
* Just for completeness, the /*+ APPEND */ hint causes the insert to
allocate a batch of empty blocks below the high water mark and effectively
extend the table. That requires an exclusive lock on the table. And exclusive
locks can be bad for concurrency.
On 10/27/18 6:49 PM, Eriovaldo Andrietta wrote:
Hello,
I using Oracle 12c R2.
I have this query that shows blocked sid:
Query 1 :
select * from gv$lock where sid in
(select sid from gv$session where blocking_session is not null);
and here , the query shows the blocker sid. These sessions are IDLE.
Query 2
select * from gv$lock where sid in
(select blocking_session from gv$session where blocking_session is not null);
My doubt is : what is the query that caused the lock that ran in the blocker
sessions?
I tried the query below, but did not get any query using the locked table.
select * from
(
select sql_id
,to_char(last_active_time,'dd-hh24:mi:ss') last_active
,executions
,elapsed_time/1000000 elap_tot
,decode(executions,0,elapsed_time,(elapsed_time/executions))/1000000
elap_exec
,decode(executions,0,disk_reads,(disk_reads/executions)) disk_exec
,decode(executions,0,buffer_gets,(buffer_gets/executions)) buffer_exec
,tot
,sql_text
from (select s.sql_id
,substr(s.sql_text,1,225) sql_text
,max(last_active_time) last_active_time
,sum(executions) executions
,sum(elapsed_time) elapsed_time
,sum(disk_reads) disk_reads
,sum(buffer_gets) buffer_gets
,count(*) tot
from gv$sql s
,gv$open_cursor o
where s.inst_id = o.inst_id
and s.sql_id = o.sql_id
and o.user_name = s.parsing_schema_name
and o.sid=&vSid
and o.inst_id = nvl('&vInstId',1)
and s.parsing_schema_name <> 'SYS'
group by s.sql_id
,substr(s.sql_text,1,225))
order by to_char(last_active_time,'yyyymmddhh24mi'), elap_exec
)
where elap_exec > &vElap;
What is the way to find out the query that caused the lock (in sessions showed
in the Query 2 above)
Regards
Eriovaldo
--
Mladen Gogala
Database Consultant
Tel: (347) 321-1217
