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PostgreSQL代码分析,查询优化部分,canonicalize_qual

PostgreSQL代码分析,查询优化部分。

张大明白的blog:http://blog.csdn.net/shujiezhang

相关博文:PostgreSQL代码分析,查询优化部分,process_duplicate_ors



/*
 * canonicalize_qual
 *	  Convert a qualification expression to the most useful form.
 *
 * The name of this routine is a holdover from a time when it would try to
 * force the expression into canonical AND-of-ORs or OR-of-ANDs form.
 * Eventually, we recognized that that had more theoretical purity than
 * actual usefulness, and so now the transformation doesn't involve any
 * notion of reaching a canonical form.
 *
 * NOTE: we assume the input has already been through eval_const_expressions
 * and therefore possesses AND/OR flatness.  Formerly this function included
 * its own flattening logic, but that requires a useless extra pass over the
 * tree.
 *
 * Returns the modified qualification.
 */
/*
 * 对WHERE语句或ON语句中的条件进行规范化,从集合的角度对AND和OR两个操作做合并分解。
 */
Expr *
canonicalize_qual(Expr *qual)
{
	Expr	   *newqual;

	/* Quick exit for empty qual */
	if (qual == NULL)
		return NULL;

	/*
	 * Pull up redundant subclauses in OR-of-AND trees.  We do this only
	 * within the top-level AND/OR structure; there's no point in looking
	 * deeper.
	 */
	/*
	 * 查找重复的OR操作,即化简条件语句。
	 * 假设WHERE条件为:
	 *			(A=1 AND B=1) OR (A=1 AND C=1)
	 * 可以化简为:
	 *			A=1 AND (B=1 OR C=1)
	 *
	 * 另外,这个函数中做了将树状的AND或OR语句平面化(flatten,或拉平)的工作,
	 * 这两个工作主要体现在pull_ands()和pull_ors()两个函数中。
	 */
	newqual = find_duplicate_ors(qual);

	return newqual;
}


/*
 * find_duplicate_ors
 *	  Given a qualification tree with the NOTs pushed down, search for
 *	  OR clauses to which the inverse OR distributive law might apply.
 *	  Only the top-level AND/OR structure is searched.
 *
 * Returns the modified qualification.	AND/OR flatness is preserved.
 */
static Expr *
find_duplicate_ors(Expr *qual)
{
	/*
	 * “分支一:or_clause”
	 *
	 * 如果WHERE表达式中的主操作是OR,例如是如下形式:
	 *			A=1 OR B=1 OR (C=1 AND D=1)
	 * 那么参数qual指针实际指向一个BoolExpr结构体。
typedef struct BoolExpr
{
	Expr		xpr; = 略
	BoolExprType boolop; = OR_EXPR,即对下面的3个进行OR操作
	List	   *args; = 3个,分别是A=1和 B=1和(C=1 AND D=1)
} BoolExpr;
	 *
	 * 这里的args是一个list,其中的3个元素的类型分别如下:
	 * 第一个是比较操作,类型是OpExpr
	 * 第二个是比较操作,类型是OpExpr
	 * 第三个是AND操作,是一个AND_EXPR类型的BoolExpr,递归调用时会处理
	 *
	 * 张大明白的blog:http://blog.csdn.net/shujiezhang
	 */
	if (or_clause((Node *) qual))
	{
		List	   *orlist = NIL;
		ListCell   *temp;

		/* Recurse */
		/*
		 * 对BoolExpr中的三个条件分别进行递归处理。
		 * 在这里需要重点关注上例中的AND_EXPR类型的BoolExpr,因为在递归调用中,他将
		 * 触发下一个分支(分支二)。
		 */
		foreach(temp, ((BoolExpr *) qual)->args)
			orlist = lappend(orlist, find_duplicate_ors(lfirst(temp)));

		/*
		 * Don't need pull_ors() since this routine will never introduce an OR
		 * where there wasn't one before.***这个原因没理解。***
		 */
		/* 详情见《PostgreSQL代码分析,查询优化部分,process_duplicate_ors》博文*/
		return process_duplicate_ors(orlist);
	}
	/*
	 * “分支二:and_clause”
	 *
	 *  这里主要做了两个操作:
	 *	1) 如果子语句中有OR类型的子语句,则递归调用find_duplicate_ors,因为 子OR语句中
	 *      或许也能提取公共项。
	 *	2) 对AND操作进行拉平。
	 */
	else if (and_clause((Node *) qual))
	{
		List	   *andlist = NIL;
		ListCell   *temp;

		/* Recurse */
		/* 
		 * 子语句中存在一系列OR的情况。
		 * 例如对于:
		 *		A=1 AND ((B=1 AND C=1) OR (B=1 AND D=1))
		 * 这里的qual指针指向一个AND_EXPR类型的BoolExpr结构体,则
		 *
typedef struct BoolExpr
{
	Expr		xpr; = 略
	BoolExprType boolop; = AND_EXPR,即对下面的2个进行AND操作
	List	   *args; = 2个,分别是“A=1”和 “((B=1 AND C=1) OR (B=1 AND D=1))”
} BoolExpr;
		 *
		 * 对于其中的“((B=1 AND C=1) OR (B=1 AND D=1))”,在递归调用中,
		 * 会进入“分支一:or_clause”,进而转换为:
		 *		B=1 AND (C=1 OR D=1)
		 *
		 * 张大明白的blog:http://blog.csdn.net/shujiezhang
		 */
		foreach(temp, ((BoolExpr *) qual)->args)
			andlist = lappend(andlist, find_duplicate_ors(lfirst(temp)));

		/* Flatten any ANDs introduced just below here */
		/* 
		 * 拉平。
		 *
		 * 因为主语句是AND类型,子语句也是AND类型,所以可以直接把子语句拉到父节点。
		 * 
		 */
		andlist = pull_ands(andlist);
		/* The AND list can't get shorter, so result is always an AND */
		return make_andclause(andlist);
	}
	else
		return qual;
}

PostgreSQL代码分析,查询优化部分。

张大明白的blog:http://blog.csdn.net/shujiezhang

相关博文:PostgreSQL代码分析,查询优化部分,process_duplicate_ors


PostgreSQL代码分析,查询优化部分,canonicalize_qual