“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.”
Maybe I’m misunderstanding, but a select over a DB link does not lock the
table/row in any way. You do start a transaction and acquire a DX type lock
but that does not effect the table.
A select statement will put a lock on the rows selected when the ‘for
update’ clause is used. These can be very common for some applications and
not used at all in others.
I enjoyed the anecdote about /*+append*/, note that thankfully Oracle is
sensible enough for the hint not to apply for insert..values, only
insert..select statements. You would use insert_values hint if you really
wanted to use direct path insert with insert..values, this could be
legitimate in cases using huge array binds but I’d be skepticle.
Regards,
Andy
On Mon, 29 Oct 2018 at 02:42, Mladen Gogala <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
