Read by other session

原文地址：Read<wbr>by<wbr>other<wbr>session</wbr></wbr></wbr> 作者：顾问

Applies to:

Oracle Server - Enterprise Edition - Version: 10.1.0.2 to 10.2.0.1 - Release: 10.1 to 10.2
Information in this document applies to any platform.

Goal

What does "Read By Other Session" wait event mean ?

Solution

When information is requested from the database, Oracle will first read the data from disk into the database buffer cache. If two or more sessions request the same information, the first session will read the data into the buffer cache while other sessions wait. In previous versions, this wait was classified under the "buffer busy waits" event. However, in Oracle 10.1 and higher, this wait time is now broken out into the "read by other session" wait event. Excessive waits for this event are typically due to several processes repeatedly reading the same blocks, e.g. many sessions scanning the same index or performing full table scans on the same table. Tuning this issue is a matter of finding and eliminating this contention.

When a session is waiting on this event, an entry will be seen in the v$session_wait system view giving more information on the blocks being waited for:

SELECT p1 "file#", p2 "block#", p3 "class#"
FROM v$session_wait
WHERE event = 'read by other session';

If information collected from the above query repeatedly shows that the same block (or range of blocks) is experiencing waits, this indicates a "hot" block or object.

The following query will give the name and type of the object:

SQL> SELECT relative_fno, owner, segment_name, segment_type
FROM dba_extents
WHERE file_id = &file
AND &block BETWEEN block_id AND block_id + blocks - 1