这里把规范谓词表达式的部分就整理完了,阅读的顺序如下:
一、PostgreSQL代码分析,查询优化部分,canonicalize_qual
二、PostgreSQL代码分析,查询优化部分,pull_ands()和pull_ors()
三、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; }