如何查找热点块

来源：互联网发布：qt 关闭release优化编辑：程序博客网时间：2024/04/29 19:47

V$LATCH字段说明：

gets表示总共有这么多次请求，misses表示请求失败的次数(加锁不成功)，而sleeps 表示请求失败休眠的次数，通过sleeps我们可以大体知道数据库中latch的竞争是否严重，这也间接的表征了热点块的问题是否严重

查看与热块有关的latch信息（cache buffer%）：

SQL> SELECT latch#, NAME, gets, misses, sleeps  2    FROM v$latch  3   WHERE NAME LIKE 'cache buffer%';     LATCH# NAME                                                     GETS     MISSES     SLEEPS---------- -------------------------------------------------- ---------- ---------- ----------       123 cache buffer handles                                      759          0          0       117 cache buffers lru chain                                 96611         12         12       122 cache buffers chains                                  2253780          4          4

V$LATCH_CHILDREN包含子latch的信息，如果子latch的latch#列值相同，则说明他们有相同的父latch。

查看子latch信息（cache buffers chains）：

SQL> SELECT addr, LATCH#, CHILD#, gets, misses, sleeps  2    FROM v$latch_children  3   WHERE NAME = 'cache buffers chains'  4     AND rownum < 21; ADDR         LATCH#     CHILD#       GETS     MISSES     SLEEPS-------- ---------- ---------- ---------- ---------- ----------290C07C4        122          1       2426          0          0290C0940        122          2        589          0          0290C0ABC        122          3       2701          0          0290C0C38        122          4       1844          0          0290C0DB4        122          5       1214          0          0290C0F30        122          6        652          0          0290C10AC        122          7       4897          0          0290C1228        122          8       3474          0          0290C13A4        122          9        977          0          0290C1520        122         10       4210          0          0290C169C        122         11       3392          0          0290C1818        122         12       2913          0          0290C1994        122         13        385          0          0290C1B10        122         14        822          0          0290C1C8C        122         15       2333          0          0290C1E08        122         16       4714          0          0290C1F84        122         17       1001          0          0290C2100        122         18        419          0          0290C227C        122         19       1735          0          0290C23F8        122         20       1105          0          0 20 rows selected

根据v$latch_child.addr关联到对应的x$bh.hladdr(这是buffer header中记录的当前buffer所处的latch地址)，通过x$bh可以获得块的文件编号和block编号。

SQL> SELECT dbarfil, dbablk  2    FROM x$bh  3   WHERE hladdr IN (SELECT addr  4                      FROM (SELECT addr, LATCH#, CHILD#, gets, misses, sleeps  5                              FROM v$latch_children  6                             WHERE NAME = 'cache buffers chains'  7                             ORDER BY sleeps DESC)  8                     WHERE rownum < 11);    DBARFIL     DBABLK---------- ----------         1      55375         4       8500         3        756         3       5094         2       1455         1      50338         1       2154         3      30656         1       6492         3       4395    ……104 rows selected SQL> SQL> SELECT dbarfil, dbablk  2    FROM x$bh  3   WHERE hladdr IN  4         (SELECT addr  5            FROM (SELECT addr FROM v$latch_children ORDER BY sleeps DESC)  6           WHERE rownum < 11);    DBARFIL     DBABLK---------- ----------         1      55375         4       8500         3        756         3       5094         2       1455         1      50338         1       2154         3      30656         1       6492         3       4395         2        769  ……37 rows selected

知道了文件编号和block编号，可以通过dba_extents获取相关的segment。

SQL> SELECT DISTINCT a.owner, a.segment_name, a.segment_type  2    FROM dba_extents a,  3         (SELECT dbarfil, dbablk  4            FROM x$bh  5           WHERE hladdr IN  6                 (SELECT addr  7                    FROM (SELECT addr, LATCH#, CHILD#, gets, misses, sleeps  8                            FROM v$latch_children  9                           WHERE NAME = 'cache buffers chains' 10                           ORDER BY sleeps DESC) 11                   WHERE rownum < 11)) b 12   WHERE a.RELATIVE_FNO = b.dbarfil 13     AND a.BLOCK_ID <= b.dbablk 14     AND a.block_id + a.blocks > b.dbablk 15     AND a.owner = 'OCP'; OWNER                          SEGMENT_NAME                                                                     SEGMENT_TYPE------------------------------ -------------------------------------------------------------------------------- ------------------OCP                            T                                                                                TABLESQL> SELECT DISTINCT a.owner, a.segment_name, a.segment_type  2    FROM dba_extents a,  3         (SELECT dbarfil, dbablk  4            FROM x$bh  5           WHERE hladdr IN  6                 (SELECT addr  7                    FROM (SELECT addr FROM v$latch_children ORDER BY sleeps DESC)  8                   WHERE rownum < 11)) b  9   WHERE a.RELATIVE_FNO = b.dbarfil 10     AND a.BLOCK_ID <= b.dbablk 11     AND a.block_id + a.blocks > b.dbablk 12     AND a.owner = 'OCP'; OWNER                          SEGMENT_NAME                                                                     SEGMENT_TYPE------------------------------ -------------------------------------------------------------------------------- ------------------OCP                            T                                                                                TABLESQL>

在v$sqlarea或者v$sqltext里找到与热点块有关的sql进行优化。

其实也就是下面的语句：

SELECT sql_text
FROM v$sqltext a
WHERE a.sql_text LIKE '%t%'
ORDER BY a.hash_value, a.address, a.piece;

SELECT sql_text  FROM v$sqltext a,       (SELECT DISTINCT a.owner, a.segment_name, a.segment_type          FROM dba_extents a,               (SELECT dbarfil, dbablk                  FROM x$bh                 WHERE hladdr IN (SELECT addr                                    FROM (SELECT addr,                                                 LATCH#,                                                 CHILD#,                                                 gets,                                                 misses,                                                 sleeps                                            FROM v$latch_children                                           WHERE NAME = 'cache buffers chains'                                           ORDER BY sleeps DESC)                                   WHERE rownum < 11)) b         WHERE a.RELATIVE_FNO = b.dbarfil           AND a.BLOCK_ID <= b.dbablk           AND a.block_id + a.blocks > b.dbablk           AND a.owner = 'OCP') b WHERE a.sql_text LIKE '%' || b.segment_name || '%'   AND b.segment_type = 'TABLE' ORDER BY a.hash_value, a.address, a.piece;SELECT sql_text  FROM v$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 a.hash_value, a.address, a.piece;

SELECT dbarfil, dbablk FROM x$bh ORDER BY tch DESC

x$bh.tch （touch count）大的block可能暗示着在当前某个周期内被访问次数比较多。

查看访问次数比较多的块所在对象：

SELECT t.owner, t.object_name, t2.dbarfil, t2.dbablk,t2.tch   FROM dba_objects t, x$bh t2 WHERE t.data_object_id = t2.obj   AND owner = 'OCP'   AND t2.ts# > 0  --ts#表示表空间块号,是什么意思呢？ ORDER BY t2.tch DESC

查看访问次数比较多的对象：

SELECT *  FROM (SELECT o.owner, o.object_name, SUM(tch) TouchTime          FROM x$bh b, dba_objects o         WHERE b.obj = o.data_object_id AND o.owner='OCP'         AND b.ts#>0  --ts#表示表空间块号，,是什么意思呢？         GROUP BY o.owner, o.object_name         ORDER BY SUM(tch) DESC) WHERE rownum <= 10

除了优化sql外，当然对于热点的表或者索引来说，如果小的话，我们可以考虑cache在内存中，这样可能降低物理读提高sql运行速度(这并不会减少cache buffer chains的访问次数)，对于序列，我们可以对序列多设置一些cache。如果是并行服务器环境中的索引对象，并且这个索引是系列递增类型，我们可以考虑反向索引。