When a session needs to read data from disk into the Oracle buffer cache, it may have to wait for another session to finish doing the same thing. Time spent doing this is recorded as a read by other session event. I decided to reproduce this in an Oracle 12 database. First, in session 1, in red, I set up a user called Fred to create a table:
SQL> conn / as sysdba
Connected.
SQL> create user fred
2 identified by bloggs
3 default tablespace users
4 quota unlimited on users
5 /
User created.
SQL> grant create session, create table,
2 select any dictionary, alter system to fred
3 /
Grant succeeded.
SQL>
Still in session 1, Fred then created a table, added some data and checked that all the rows were in the same block:
SQL> conn fred/bloggs
Connected.
SQL> create table t1 (c1 number)
2 /
Table created.
SQL> begin
2 for i in 1..200 loop
3 insert into t1 values(1);
4 end loop;
5 end;
6 /
PL/SQL procedure successfully completed.
SQL> select dbms_rowid.rowid_block_number(rowid), count(*)
2 from t1
3 group by dbms_rowid.rowid_block_number(rowid)
4 order by 1
5 /
DBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID) COUNT(*)
------------------------------------ ----------
223 200
SQL>
Then he started updating the data. After each update, he did a commit so that other sessions could see the changed data and a checkpoint to write the changes to disk:
SQL> begin
2 while (1=1) loop
3 update t1 set c1 = c1 + 1;
4 commit;
5 execute immediate 'alter system checkpoint';
6 end loop;
7 end;
8 /
In 5 separate sessions in blue (sessions 2 through 6 inclusive), Fred repeatedly flushed the Oracle buffer cache and read the table into it:
SQL> conn fred/bloggs
Connected.
SQL> declare
2 grand_total number;
3 begin
4 while (1=1) loop
5 execute immediate 'alter system flush buffer_cache';
6 select sum(c1) into grand_total from t1;
7 end loop;
8 end;
9 /
In a 7th session,in green, Fred read the table into the buffer cache 10000 times. While he was doing this he had to wait for sessions 2 through 6, which were doing the same thing:
SQL> conn fred/bloggs
Connected.
SQL> declare
2 grand_total number;
3 begin
4 for i in 1..10000 loop
5 execute immediate 'alter system flush buffer_cache';
6 select sum(c1) into grand_total from t1;
7 end loop;
8 end;
9 /
PL/SQL procedure successfully completed.
SQL>
So, when he finished, the time spent waiting on the read by other session event could clearly be seen:
SQL> select event, time_waited/100
2 from v$session_event
3 where sid = (select distinct sid from v$mystat)
4 /
EVENT TIME_WAITED/100
----------------------------------- ---------------
Disk file operations I/O .03
latch: cache buffers chains .01
buffer busy waits 0
read by other session 33.36
db file sequential read 7.35
db file scattered read 22.07
db file parallel read .2
latch: In memory undo latch 0
latch: row cache objects 0
library cache: mutex X 0
SQL*Net message to client 0
SQL*Net message from client .02
events in waitclass Other 405.87
13 rows selected.