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Oracle的硬解析和软解析

原创 Linux操作系统 作者:chinagjj2008 时间:2009-04-13 23:29:40 0 删除 编辑
说到软解析(soft prase)和硬解析(hard prase),就不能不说一下Oracle对sql的处理过程。当你发出一条sql语句交付Oracle,在执行和获取结果前,Oracle对此sql将进行几个步骤的处理过程:

  1、语法检查(syntax check)

  检查此sql的拼写是否语法。

  2、语义检查(semantic check)

  诸如检查sql语句中的访问对象是否存在及该用户是否具备相应的权限。

  3、对sql语句进行解析(prase)

  利用内部算法对sql进行解析,生成解析树(parse tree)及执行计划(execution plan)。

  4、执行sql,返回结果(execute and return)

  其中,软、硬解析就发生在第三个过程里。

  Oracle利用内部的hash算法来取得该sql的hash值,然后在library cache里查找是否存在该hash值;

  假设存在,则将此sql与cache中的进行比较;

  假设“相同”,就将利用已有的解析树与执行计划,而省略了优化器的相关工作。这也就是软解析的过程。

  诚然,如果上面的2个假设中任有一个不成立,那么优化器都将进行创建解析树、生成执行计划的动作。这个过程就叫硬解析。

  创建解析树、生成执行计划对于sql的执行来说是开销昂贵的动作,所以,应当极力避免硬解析,尽量使用软解析。

  这就是在很多项目中,倡导开发设计人员对功能相同的代码要努力保持代码的一致性,以及要在程序中多使用绑定变量的原因。

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大家都在说在Sql中使用了Bind Var(绑定变量)会提高不少性能,那他到底是如何提高性能的呢?

使用了Bind Var能提高性能主要是因为这样做可以尽量避免不必要的硬分析(Hard Parse)而节约了时间,同时节约了大量的CPU资源。


当一个Client提交一条Sql给Oracle后,Oracle首先会对其进行解析(Parse),然后将解析结果提交给优化器(Optimiser)来进行优化而取得Oracle认为的最优的Query Plan,然后再按照这个最优的Plan来执行这个Sql语句(当然在这之中如果只需要软解析的话会少部分步骤)。
但是,当Oracle接到Client提交的Sql后会首先在共享池(Shared Pool)里面去查找是否有之前已经解析好的与刚接到的这一个Sql完全相同的Sql(注意这里说的是完全相同,既要求语句上的字符级别的完全相同,又要求涉及的对象也必须完全相同)。当发现有相同的以后解析器就不再对新的Sql在此解析而直接用之前解析好的结果了。这里就节约了解析时间以及解析时候消耗的CPU资源。尤其是在OLTP中运行着的大量的短小Sql,效果就会比较明显了。因为一条两条Sql的时间可能不会有多少感觉,但是当量大了以后就会有比较明显的感觉了。

上面说到了硬解析(Hard Parse),那这个Hard Parse到底是个啥呢?
Parse主要分为三种:
1、Hard Parse (硬解析)
2、Soft Parse (软解析)
3、Soft Soft Parse(好像有些资料中并没有将这个算在其中)

Hard Parse就是上面提到的对提交的Sql完全重新从头进行解析(当在Shared Pool中找不到时候将会进行此操作),总共有一下5个执行步骤:
1:语法分析
2:权限与对象检查
3:在共享池中检查是否有完全相同的之前完全解析好的—如果存在,直接跳过4和5,运行Sql(此时算soft parse)
4:选择执行计划
5:产生执行计划

Soft Parse就如果是在Shared Pool中找到了与之完全相同的Sql解析好的结果后会跳过Hard Parse中的后面的两个步骤。

Soft Soft Parse实际上是当设置了session_cursor_cache这个参数之后,Cursor被直接Cache在当前Session的PGA中的,在解析的时候只需要对其语法分析、权限对象分析之后就可以转到PGA中查找了,如果发现完全相同的Cursor,就可以直接去取结果了,也就就是实现了Soft Soft Parse。

不过在计算解析次数的时候是只计算Hard Parse和Soft Parse的(其实Soft Soft Parse好像也并不能算是做了Parse ) ):
Soft Parse百分比计算:Round(100*(1-:hprs/:prse),2) [hprs:硬解析次数;prse:解析次数]
Parse比率计算: Round(100*(1-prse/exec) ,2) [exec:执行次数]

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quote from difference between soft parse and hard parse


-- Thanks for the question regarding "difference between soft parse and hard parse", version 8.1.7.2
originally submitted on 26-Jan-2002 19:17 Eastern US time, last updated 5-Aug-2004 13:07


Hi Tom

Can you explain briefly the difference between soft and hard parse?

你能不能给我详细介绍一下软解析与硬解析的区别啊?

and we said...

Here is a long winded answer, it is extracted in part from a new book oming out soon "beginning Oracle programming" that I collaborated on:
这是一个很长的解释,是从我与几个同事一起合著的新书<<开始学习Oracle编程>>中提取出来的.

Parsing
解析

This is the first step in the processing of any statement in Oracle. Parsing is
the act of breaking the submitted statement down into its component parts ? determining what type of statement it is (query, DML, DDL) and performing various checks on it.
这是Oracle处理所有语句的第一步,解析就是将用户提交的语句(根据一定的规则)分成不同的组件,来确定这条语句是什么类型的语句(query,查询,DML,数据操纵语言,DDL还是数据定义语言),以对这条语句执行各种不同的语法检测.

The parsing process performs two main functions:
解析过程主要有一下两个功能.

  • Syntax Check: is the statement a valid one? Does it make sense given the SQL grammar documented in the SQL Reference Manual. Does it follow all of the rules for SQL.
    1. 语法检查,这条语句是否合理? 它是否符合SQL语法文档<>的相关语法,是不是符合SQL语句的所有规则.

    o Semantic Analysis: Going beyond the syntax ? is the statement valid in light of the objects in the database (do the tables and columns referenced exist). Do you have access to the objects ? are the proper privileges in place? Are there ambiguities in the statement ? for example if there are two tables T1 and T2 and both have a column X, the query ?select X from T1, T2 where ?? is ambiguous, we don?t know which table to get X from. And so on.
    2.语义分析: 已经处理完语法检查了吗? 这条语句是否正确的引用了数据库中对象
    (是否语句中所有的表和字段都存在). 是否有访问这些对象的权限? 是不是对应的权限都有效(主要指role对应的权限吧,)? 比如是否有如下的两个表T1,T2,而这两个表有相同的字段名column X,而查询语句 ? select X from T1,T2 where ??,(没有明确指定列名),我们无法知道从哪个表去取出字段X的值,以及类似的一系列问题.

    So, you can think of parsing as basically a two step process, that of a syntax
    check to check the validity of the statement and that of a semantic check ? to ensure the statement can execute properly. The difference between the two types of checks are hard for you to see ? Oracle does not come back and say ?it failed the syntax check?, rather it returns the statement with a error code and message. So for example, this statement fails with a syntax error:
    因此,你可以认为解析基本上是由这两个步骤组成的: 检查语句有效性的语法检查和确保语句能够正常运行的语义检查.这两种检查的区别就是你所要说的硬解析了.Oracle不会特别的指出这条语句没有通过语法检查,它给你返回这条语句对应的错误代码和错误信息. 比如: 下面这条语句没有通过语法检查.

    Code: [Copy to clipboard]
    SQL> select from where 2;
    select from where 2
    *
    ERROR at line 1:
    ORA-00936: missing expression

    While this statement failed with a semantic error ? if the table NOT_A_TABLE existed and we had permission to access it, this statement would have succeeded:

    而这条语句没有通过语义检查? 如果表NOT_A_TABLE存在,并且我们具有访问这个表的权限,这条语句就会正确执行了.

    Code: [Copy to clipboard]
    SQL> select * from not_a_table;
    select * from not_a_table
    *
    ERROR at line 1:
    ORA-00942: table or view does not exist

    That is the only way to really tell the difference between a semantic and
    syntactic error ? if the statement COULD have executed given the proper objects and privileges, you had a semantic error, otherwise if the statement could not execute under any circumstances, you have a syntax error. Regardless ? Oracle will not execute the statement for you!

    只是我能用来解释语法错误和语义错误的唯一的办法了.如果在赋予合适的对象和权限的情况下,这条语句确实能够执行的话,我们称之为语义错误,否则,如果这条语句在任何条件下都不能执行的话,我们就称之为语法错误.无论如何,Oracle都不会为你运行这条语句.

    The next step in the parse operation is to see if the statement we are currently parsing has already in fact been processed by some other session. If it has ? we may be in luck here, we can skip the next two steps in the process, that of optimization and row source generation. If we can skip these next two steps in the process, we have done what is known as a Soft Parse ? a shorter process to getting our query going. If we cannot, if we must do all of the steps, we are performing what is known as a Hard Parse ? we must parse, optimize, generate the plan for the query. This distinction is very important. When developing our applications we want a very high percentage of our queries to be Soft Parsed ? to be able to skip the optimize/generate phases ? as they are very CPU intensive as well as a point of contention (serialization). If we have to Hard Parse a large percentage of our queries, our system will function slowly and in some cases ? not at all.

    解析中的下一个步骤就是检查我们现在正在解析的语句是否已经被其他的session执行过,在这一步我们可能很幸运,我们可以跳过下面的两个步骤: 语句的优化(生成执行计划) 和 生成执行编码. 如果我们能够跳过这两个步骤,我们就实现了一般意义上的软解析, 我们必须解析,优化,并为这条语句生成执行计划.这个特性(distinction)非常重要,当我们开发应用程序的时候,我们会希望大部分的语句都是软解析的,以跳过优化和生成编码的步骤,因为他们和争用(串行化,)(contention)一样,都是非常消耗CPU的操作.如果我们必须硬解析我们语句的大部分的话,在某些情况下我们的系统就会运行的很慢,并不是任何时候(在olap,dss系统应该是个例外吧)

    The way this sharing of SQL in Oracle is accomplished is via the shared pool, a piece of memory in the SGA maintained by Oracle. We covered this topic in chapter 5 but will revisit it again in the context of processing a query. After Oracle parses the query and it passes the syntax and semantic checks ? it will look in the shared pool component of the SGA to see if that same exact query has already been processed by another session. Since it has performed the semantic check it has already figured out:

    在Oracle中共享SQL的方式是通过共享池--SGA中的一块内存(由Oracle系统来维护)--实现的.我们在第五章中讨论了这个主题,并且会在查询处理的章节再次讨论这个主题.当Oracle已经解析了这条语句,并且已经通过语法和语义检查的时候,Oracle就会在SGA的shared pool(共享池)组件中查询,看看,是否已经有一个完全一样的语句已经被另外一个session执行过.因为语句已经通过了oracle的语义检查,并且oracle已经计算出:

    o Exactly what tables are involved
    具体涉及到哪些表.

    o That we have access to the tables (the proper privileges are there)
    我们拥有访问对应表的个相应权限.等等...

    And so on. Now, it can look at all of the queries in the shared pool that have already been parsed/optimized and generated to see if the work has already been done.
    现在,可以在所有已经经过解析和优化并且生成的可执行编码的SQL语句搜索,来看看对应的语句是否已经经过解析.

    Soft Parse and session_cashed_cursor parameter January 28, 2002
    Reviewer: A reader

    软解析与session_cashed_cursor参数

    can you explain what means the default value (0) of session_ casched_cursor parameter? Oracle always has a cache of sqls in sga and size of this cache is determinated through init.ora parameter shared_pool_size.

    tom : 你能不能给我解释一下,session_cashed_cursor(默认值为0)参数的含义,Oracle在SGA中始终保持一个sql语句的缓存.这个缓存的大小是由初始化参数shared_pool_size的大小决定的.

    Followup:
    The default value of ZERO means that by default, ZERO cursors will be cached for your session.

    默认值为0表示,在默认情况下,Oracle不会给你的session缓存游标.

    They will be cached in the shared pool -- but your session will have to find
    them there. Session Cached Cursors can remove the need to have to "find" them. Consider this example that shows the difference spent when soft parsing without session cached cursors vs soft parsing WITH session cached cursors:
    他们会被缓存在共享池(Shared Pool)中,但是你的session必须在共享池中去查找他们,session cached cursors可以省略掉去共享池中查找这一步.下面这个例子
    给你演示了没有缓存游标缓存了游标之后软解析之间的差别和系统消耗资源的情况

    Code: [Copy to clipboard]
    ops$[email]tkyte@ORA817DEV.US.ORACLE.COM[/email]> create table emp as select * from scott.emp;
    Table created.

    ops$[email]tkyte@ORA817DEV.US.ORACLE.COM[/email]> create table run_stats ( runid varchar2(15),
    name varchar2(80), value int );
    Table created.

    ops$[email]tkyte@ORA817DEV.US.ORACLE.COM[/email]> create or replace view stats
    2 as select 'STAT...' || a.name name, b.value
    3 from v$statname a, v$mystat b
    4 where a.statistic# = b.statistic#
    5 union all
    6 select 'LATCH.' || name, gets
    7 from v$latch;

    View created.


    ops$[email]tkyte@ORA817DEV.US.ORACLE.COM[/email]> column name format a40
    ops$[email]tkyte@ORA817DEV.US.ORACLE.COM[/email]> declare
    2 l_start number;
    3 l_cnt number;
    4 begin
    5 execute immediate 'alter session set session_cached_cursors=0';
    6 insert into run_stats select 'before', stats.* from stats;
    7
    8 l_start := dbms_utility.get_time;
    9 for i in 1 .. 1000
    10 loop
    11 execute immediate 'select count(*) from emp' into l_cnt;
    12 end loop;
    13 dbms_output.put_line( (dbms_utility.get_time-l_start) || ' hsecs' );
    14
    15 execute immediate 'alter session set session_cached_cursors=100';
    16 insert into run_stats select 'after 1', stats.* from stats;
    17
    18 l_start := dbms_utility.get_time;
    19 for i in 1 .. 1000
    20 loop
    21 execute immediate 'select count(*) from emp' into l_cnt;
    22 end loop;
    23 dbms_output.put_line( (dbms_utility.get_time-l_start) || ' hsecs' );
    24
    25 insert into run_stats select 'after 2', stats.* from stats;
    26 end;
    27 /
    45 hsecs
    35 hsecs

    PL/SQL procedure successfully completed.

    so, session cached cursors RAN faster (i ran this a couple of times, there
    were no hard parses going on. But the real good news is:

    因此,在session中缓存游标可以运行的更快(我运行了好几次,没有硬解析的发生),真正可喜的消息是:

    Code: [Copy to clipboard]
    ops$[email]tkyte@ORA817DEV.US.ORACLE.COM[/email]> select a.name, b.value-a.value run1,
    c.value-b.value run2,
    2 ( (c.value-b.value)-(b.value-a.value)) diff
    3 from run_stats a, run_stats b, run_stats c
    4 where a.name = b.name
    5 and b.name = c.name
    6 and a.runid = 'before'
    7 and b.runid = 'after 1'
    8 and c.runid = 'after 2'
    9 and (c.value-a.value) > 0
    10 and (c.value-b.value) <> (b.value-a.value)
    11 order by abs( (c.value-b.value)-(b.value-a.value))
    12 /

    NAME RUN1 RUN2 DIFF
    ---------------------------------------- ---------- ---------- ----------
    LATCH.checkpoint queue latch 3 4 1
    LATCH.redo allocation 30 31 1
    STAT...consistent gets 5088 5089 1
    STAT...deferred (CURRENT) block cleanout 2 3 1
    applications

    STAT...calls to get snapshot scn: kcmgss 5019 5018 -1
    STAT...enqueue releases 10 9 -1
    STAT...execute count 1015 1014 -1
    STAT...opened cursors cumulative 1015 1014 -1
    STAT...parse count (total) 1015 1014 -1
    STAT...session cursor cache count 0 1 1
    STAT...redo entries 28 27&nb
  • get:parse阶段查找对象的次数。
    pin:execute阶段的读取、执行对象的次数。
    reload:execute阶段未命中的库高速缓存数,可能引起对SQL语句进行重新分析。
    invalidations:分析阶段,对象修改导致解释计划被标记为无效,必须重新分析。

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