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关于热点块的查询
==== 查询当前数据库 最繁忙的 Buffer ， TCH(Touch) 表示访问次数越高，热点快竞争问题就存在 =====
SELECT *
FROM (SELECT addr, ts#, file#, dbarfil, dbablk, tch
FROM x$bh
ORDER BY tch DESC)
WHERE ROWNUM < 11;
==== 查询当前数据库最繁忙的 Buffer ，结合 dba_extents 查询得到这些热点 Buffer 来自哪些对象 =====
SELECT e.owner, e.segment_name, e.segment_type
FROM dba_extents e,
(SELECT *
FROM (SELECT addr, ts#, file#, dbarfil, dbablk, tch
FROM x$bh
ORDER BY tch DESC)
WHERE ROWNUM < 11) b
WHERE e.relative_fno = b.dbarfil
AND e.block_id <= b.dbablk
AND e.block_id + e.blocks > b.dbablk;
============= 如果在 Top 5 中发现 latch free 热点块事件时，可以从 V$latch_children 中查询具体的子 Latch 信息 ============
SELECT *
FROM (SELECT addr, child#, gets, misses, sleeps, immediate_gets igets,
immediate_misses imiss, spin_gets sgets
FROM v$latch_children
WHERE NAME = 'cache buffers chains'
ORDER BY sleeps DESC)
WHERE ROWNUM < 11;
================ 获取当前持有最热点数据块的 Latch 和 buffer 信息 ==========
SELECT b.addr, a.ts#, a.dbarfil, a.dbablk, a.tch, b.gets, b.misses, b.sleeps
FROM (SELECT *
FROM (SELECT addr, ts#, file#, dbarfil, dbablk, tch, hladdr
FROM x$bh
ORDER BY tch DESC)
WHERE ROWNUM < 11) a,
(SELECT addr, gets, misses, sleeps
FROM v$latch_children
WHERE NAME = 'cache buffers chains') b
WHERE a.hladdr = b.addr;
=============== 利用前面的 SQL 可以找到这些热点 Buffer 的对象信息 ===========
SELECT distinct e.owner, e.segment_name, e.segment_type
FROM dba_extents e,
(SELECT *
FROM (SELECT addr, ts#, file#, dbarfil, dbablk, tch
FROM x$bh
ORDER BY tch DESC)
WHERE ROWNUM < 11) b
WHERE e.relative_fno = b.dbarfil
AND e.block_id <= b.dbablk
AND e.block_id + e.blocks > b.dbablk;
================ 结合 SQL 视图可以找到操作这些对象的相关 SQL ，然后通过优化 SQL 减少数据的访问，
或者优化某些容易引起争用的操作（如 connect by 等操作）来减少热点块竞争 =================
break on hash_value skip 1
SELECT /*+ rule */ hash_value,sql_text
FROM v$sqltext
WHERE (hash_value, address) IN (
SELECT a.hash_value, a.address
FROM v$sqltext a,
(SELECT DISTINCT a.owner, a.segment_name, a.segment_type
FROM dba_extents a,
(SELECT dbarfil, dbablk
FROM (SELECT dbarfil, dbablk
FROM x$bh
ORDER BY tch DESC)
WHERE ROWNUM < 11) b
WHERE a.relative_fno = b.dbarfil
AND a.block_id <= b.dbablk
AND a.block_id + a.blocks > b.dbablk) b
WHERE a.sql_text LIKE '%' || b.segment_name || '%'
AND b.segment_type = 'TABLE')
ORDER BY hash_value, address, piece;
本文转自baoqiangwang51CTO博客，原文链接：http://blog.51cto.com/baoqiangwang/312718
，如需转载请自行联系原作者
