{ "date": "2014-11-17T13:17:07", "slug": "oracle-locks-identifiying-blocking-sessions", "link": "https://www.dbi-services.com/blog/oracle-locks-identifiying-blocking-sessions/", "title": { "rendered": "Oracle locks: Identifying blocking sessions" }, "content": { "rendered": "

By Franck Pachot

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\nWhen you have sessions blocked on locks, you probably have all information about the waiters (they call you and anyway their waiting session is visible in v$session our ASH). But you usually need to get enough information that help to identify the blocker.

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Here is a query I use to get that quickly, based on V$WAIT_CHAINS

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Here is the result I want to get:

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session                 wait event                                      minutes USER PRO\n----------------------- ----------------------------------------------- ------- ---- ---\n ABCLBP1 '831,54109@1'  SQL*Net message from client                        13.5 SYS  sql\n  ABCLBP4 '395,21891@4' enq: TX - row lock contention on TABLE             13.2 SYS  SQL\n                          \"SYS\".\"TEST_FRANCK\" on rowid AAC0aCAAnAAABSCAAA\n\n
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I have information about blocking session, waiting session, the type of lock (here TX – row lock) and because it is a row lock I want to know the ROWID of the locked row.

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Here is the query I used to get it:

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column \"wait event\" format a50 word_wrap\ncolumn \"session\" format a25\ncolumn \"minutes\" format 9999D9\ncolumn CHAIN_ID noprint\ncolumn N noprint\ncolumn l noprint\nwith w as (\nselect\n chain_id,rownum n,level l\n ,lpad(' ',level,' ')||(select instance_name from gv$instance where inst_id=w.instance)||' '''||w.sid||','||w.sess_serial#||'@'||w.instance||'''' \"session\"\n ,lpad(' ',level,' ')||w.wait_event_text ||\n   case\n   when w.wait_event_text like 'enq: TM%' then\n    ' mode '||decode(w.p1 ,1414332418,'Row-S' ,1414332419,'Row-X' ,1414332420,'Share' ,1414332421,'Share RX' ,1414332422,'eXclusive')\n     ||( select ' on '||object_type||' \"'||owner||'\".\"'||object_name||'\" ' from all_objects where object_id=w.p2 )\n   when w.wait_event_text like 'enq: TX%' then\n    (\n     select ' on '||object_type||' \"'||owner||'\".\"'||object_name||'\" on rowid '\n     ||dbms_rowid.rowid_create(1,data_object_id,relative_fno,w.row_wait_block#,w.row_wait_row#)\n     from all_objects ,dba_data_files where object_id=w.row_wait_obj# and w.row_wait_file#=file_id\n    )\n   end \"wait event\"\n , w.in_wait_secs/60 \"minutes\"\n , s.username , s.program\n from v$wait_chains w join gv$session s on (s.sid=w.sid and s.serial#=w.sess_serial# and s.inst_id=w.instance)\n connect by prior w.sid=w.blocker_sid and prior w.sess_serial#=w.blocker_sess_serial# and prior w.instance = w.blocker_instance\n start with w.blocker_sid is null\n)\nselect * from w where chain_id in (select chain_id from w group by chain_id having max(\"minutes\") >= 1 and max(l)>1 )\norder by n\n/\n
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This query retrieves the wait chains where a session is waiting for more than one minute on a table lock (TM) or row lock (TX) .

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When it is a table lock (TM), I get the locked object_id from the P2 parameter, in order to know the table name.

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When it is a row lock, I get the table and rowid from V$SESSION. Note that I have to join with dba_data_files in order to convert the absolute file_id to a relative one, and to join to dba_objects in order to convert the object_id to the data_object_id one – in order to built the ROWID.

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More information about ROWID, relative file number and data object id in my previous post: From 8.0 extended rowid to 12c pluggable db: Why Oracle Database is still a great software

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