PostgreSQL查询优化逻辑优化之其他

上一节我们介绍了PostgreSQL的子查询优化，子查询优化把一部分可以优化的子查询上拉到主查询成为join。

preprocess_expression

将表达式(目标列，where，join，having)简化表达式

static Node *
preprocess_expression(PlannerInfo *root, Node *expr, int kind)
{
    /*
     * If the query has any join RTEs, replace join alias variables with
     * base-relation variables.  We must do this before sublink processing,
     * else sublinks expanded out from join aliases would not get processed.
     * We can skip it in non-lateral RTE functions, VALUES lists, and
     * TABLESAMPLE clauses, however, since they can't contain any Vars of the
     * current query level.
     */
    if (root->hasJoinRTEs &&
    	!(kind == EXPRKIND_RTFUNC ||
    	  kind == EXPRKIND_VALUES ||
    	  kind == EXPRKIND_TABLESAMPLE))
    	expr = flatten_join_alias_vars(root, expr);

    //简化常量表达式，特别处理and和or
    expr = eval_const_expressions(root, expr);

    //标准化条件表达式(where和having)
    if (kind == EXPRKIND_QUAL)
    {
    	expr = (Node *) canonicalize_qual((Expr *) expr);
    }

    /* 求解子链接的表达式，对于不能上拉的子链接会递归调用subquery_planner */
    if (root->parse->hasSubLinks)
    	expr = SS_process_sublinks(root, expr, (kind == EXPRKIND_QUAL));

    /* 给相关子查询替换变量SubPlan */
    if (root->query_level > 1)
    	expr = SS_replace_correlation_vars(root, expr);

    /*把where和having中的and表达式转换为隐含形式*/
    if (kind == EXPRKIND_QUAL)
    	expr = (Node *) make_ands_implicit((Expr *) expr);

    return expr;
}

eval_const_expressions

canonicalize_qual

使用OR分配律简化逻辑表达式，得到顶层全是AND的表达式。例如：

((A AND B) OR (A AND C)) -> (A AND (B OR C))

reduce_outer_joins

void
reduce_outer_joins(PlannerInfo *root)
{
	reduce_outer_joins_state *state;

	/*
	 * To avoid doing strictness checks on more quals than necessary, we want
	 * to stop descending the jointree as soon as there are no outer joins
	 * below our current point.  This consideration forces a two-pass process.
	 * The first pass gathers information about which base rels appear below
	 * each side of each join clause, and about whether there are outer
	 * join(s) below each side of each join clause. The second pass examines
	 * qual clauses and changes join types as it descends the tree.
	 */
	state = reduce_outer_joins_pass1((Node *) root->parse->jointree);

	/* planner.c shouldn't have called me if no outer joins */
	if (state == NULL || !state->contains_outer)
		elog(ERROR, "so where are the outer joins?");

	reduce_outer_joins_pass2((Node *) root->parse->jointree,
							 state, root, NULL, NIL, NIL);
}

static void
reduce_outer_joins_pass2(Node *jtnode,
						 reduce_outer_joins_state *state,
						 PlannerInfo *root,
						 Relids nonnullable_rels,
						 List *nonnullable_vars,
						 List *forced_null_vars)
{
    else if (IsA(jtnode, JoinExpr))
    {
    	switch (jointype)
    	{
        	case JOIN_INNER:
        		break;
        	case JOIN_LEFT://优化为内连接
        		if (bms_overlap(nonnullable_rels, right_state->relids))
        			jointype = JOIN_INNER;
        		break;
        	case JOIN_RIGHT://优化为内连接
        		if (bms_overlap(nonnullable_rels, left_state->relids))
        			jointype = JOIN_INNER;
        		break;
        	case JOIN_FULL:
        		if (bms_overlap(nonnullable_rels, left_state->relids))
        		{
        			if (bms_overlap(nonnullable_rels, right_state->relids))
        				jointype = JOIN_INNER;//优化为内连接
        			else
        				jointype = JOIN_LEFT;//优化为左外连接
        		}
        		else
        		{
        			if (bms_overlap(nonnullable_rels, right_state->relids))
        				jointype = JOIN_RIGHT;//优化为右外连接
        		}
        		break;
        	case JOIN_SEMI:
        	case JOIN_ANTI:
        		break;
        	default:
        		elog(ERROR, "unrecognized join type: %d",
        			 (int) jointype);
        		break;
    	}

    	/*剩余的右外连接优化为左外连接*/
    	if (jointype == JOIN_RIGHT)
    	{
    	}

    	/*左外连接转换为反半连接*/
    	if (jointype == JOIN_LEFT)
    	{
    	}

    	/* 递归优化之树 */
    	if (left_state->contains_outer || right_state->contains_outer)
    	{
    		if (left_state->contains_outer)
    		{
    			reduce_outer_joins_pass2(j->larg, left_state, root,
						 pass_nonnullable_rels,
						 pass_nonnullable_vars,
						 pass_forced_null_vars);
    		}

    		if (right_state->contains_outer)
    		{
    			reduce_outer_joins_pass2(j->rarg, right_state, root,
						 pass_nonnullable_rels,
						 pass_nonnullable_vars,
						 pass_forced_null_vars);
    		}
    		bms_free(local_nonnullable_rels);
    	}
    }
    else
    	elog(ERROR, "unrecognized node type: %d",
    		 (int) nodeTag(jtnode));
}

消除外连接，不是所有的外连接都能转换为内连接，只有基于"空值拒绝"的才可以。其实SQL还是外连接，只是这样可以通过调整多表连接顺序优化。

例如：left join，join后右边的列被非空条件排除

select * from x left join y on (x.x = y.y) where y.y is not null;

优化后只有FULL JOIN