Greenplum 函数内嵌套查询在query中调用的替代方案

背景

在使用数据库过程中，用户可能会封装一些函数，用来返回一些映射值。

例如，有一张表存储了学号和学生名字，写一个函数，根据学号，返回学生名字。

SQL如下 :

create table tab(id int primary key, std_name name);

create or replace function f1(int) returns name as $$
  select std_name from tab where id=$1;
$$ language sql strict volatile;

dwh=> insert into tab values (1,'abc');
INSERT 0 1

dwh=> select f1(1);
 f1
-----
 abc
(1 row)

使用Greenplum时，可能需要注意，因为如果函数中有QUERY，但是这个函数又在QUERY中调用的话，可能会报错。

目前执行计划还不支持这种查询：

dwh=> select f1(id) from tab;
ERROR:  XX000: function cannot execute on segment because it accesses relation "public.tab" (functions.c:155)  (seg0 slice1 e45f16674.cloud.nu17:3017 pid=20659) (cdbdisp.c:1326)
DETAIL:  SQL function "f1" during startup
LOCATION:  cdbdisp_finishCommand, cdbdisp.c:1326

出错原因，在检查segment节点query tree时，如果发现树中还有非catalog类的对象，则报错。
可能是master节点对这种QUERY的处理没有彻底。

/**
 * Walker for querytree_safe_for_segment.
 */
bool querytree_safe_for_segment_walker(Node *expr, void *context)
{
        Assert(context == NULL);

        if (!expr)
        {
                /**
                 * Do not end recursion just because we have reached one leaf node.
                 */
                return false;
        }

        switch(nodeTag(expr))
        {
                case T_Query:
                        {
                                Query *q = (Query *) expr;

                                if (!allow_segment_DML &&
                                        (q->commandType != CMD_SELECT
                                         || q->intoClause != NULL
                                         || q->resultRelation > 0))
                                {
                                        elog(ERROR, "function cannot execute on segment because it issues a non-SELECT statement");
                                }

                                ListCell * f = NULL;
                                foreach(f,q->rtable)
                                {
                                        RangeTblEntry *rte = (RangeTblEntry *) lfirst(f);

                                        if (rte->rtekind == RTE_RELATION)
                                        {
                                                Assert(rte->relid != InvalidOid);

                                                Oid namespaceId = get_rel_namespace(rte->relid);

                                                Assert(namespaceId != InvalidOid);

                                                if (!(IsSystemNamespace(namespaceId) ||
                                                          IsToastNamespace(namespaceId) ||
                                                          IsAoSegmentNamespace(namespaceId)))
                                                {
                                                        elog(ERROR, "function cannot execute on segment because it accesses relation \"%s.%s\"",
                                                                        quote_identifier(get_namespace_name(namespaceId)), quote_identifier(get_rel_name(rte->relid)));
                                                }
                                        }
                                }
                                query_tree_walker(q, querytree_safe_for_segment_walker, context, 0);
...

使用with也不能解决这个问题，因为不会等收到master节点后再调用f1

dwh=> with t as (select * from tab) select f1(id) from t;
ERROR:  XX000: function cannot execute on segment because it accesses relation "public.tab" (functions.c:155)  (seg0 slice1 e45f16674.cloud.nu17:3017 pid=26324) (cdbdisp.c:1326)
DETAIL:  SQL function "f1" during startup
LOCATION:  cdbdisp_finishCommand, cdbdisp.c:1326

dwh=> with t as (select * from tab) select * from t;
 id | std_name
----+----------
  1 | abc
  2 | abc
(2 rows)

聚合后还是不行
dwh=> with t as (select array_agg(id) ag from tab) select unnest(ag) from t;
 unnest
--------
      2
      1
(2 rows)

dwh=> with t as (select array_agg(id) ag from tab) select f1(unnest(ag)) from t;
ERROR:  XX000: query plan with multiple segworker groups is not supported (cdbdisp.c:302)
HINT:  likely caused by a function that reads or modifies data in a distributed table
CONTEXT:  SQL function "f1" statement 1
LOCATION:  cdbdisp_dispatchToGang, cdbdisp.c:302

怎么解决这个问题呢？
可以用子查询(括号中的部分)替代 function 。

dwh=> select *, (select std_name from tab where tab.id=t.id) from tab t;
 id | std_name | ?column?
----+----------+----------
  1 | abc      | abc
(1 row)