Fix describe on PostgreSQL views with rules

[jwangnz]

This pull request fixes the issue that the Postgres describe function might not work on views with rules.

Steps to reproduce this issue

Create a table, a view, and a rule as follows:

create table t_test (id bigint);
create table t_test_d (id bigint);
create view v_test as select id from t_test;
create rule r1 as on insert to v_test do also (delete from t_test_d where id = new.id);
explain (verbose, format json) insert into v_test (id) values (1) returning id;

Run the explain:

explain (verbose, format json) insert into v_test (id) values (1) returning id;

The output is an array of multiple Explain struct:

                   QUERY PLAN                     
---------------------------------------------------
 [                                                +
   {                                              +
     "Plan": {                                    +
       "Node Type": "ModifyTable",                +
       "Operation": "Insert",                     +
       "Parallel Aware": false,                   +
       "Async Capable": false,                    +
       "Relation Name": "t_test",                 +
       "Schema": "public",                        +
       "Alias": "t_test",                         +
       "Startup Cost": 0.00,                      +
       "Total Cost": 0.01,                        +
       "Plan Rows": 1,                            +
       "Plan Width": 8,                           +
       "Output": ["t_test.id"],                   +
       "Plans": [                                 +
         {                                        +
           "Node Type": "Result",                 +
           "Parent Relationship": "Outer",        +
           "Parallel Aware": false,               +
           "Async Capable": false,                +
           "Startup Cost": 0.00,                  +
           "Total Cost": 0.01,                    +
           "Plan Rows": 1,                        +
           "Plan Width": 8,                       +
           "Output": ["'1'::bigint"]              +
         }                                        +
       ]                                          +
     }                                            +
   },                                             +
   {                                              +
     "Plan": {                                    +
       "Node Type": "ModifyTable",                +
       "Operation": "Delete",                     +
       "Parallel Aware": false,                   +
       "Async Capable": false,                    +
       "Relation Name": "t_test_d",               +
       "Schema": "public",                        +
       "Alias": "t_test_d",                       +
       "Startup Cost": 0.00,                      +
       "Total Cost": 38.25,                       +
       "Plan Rows": 0,                            +
       "Plan Width": 0,                           +
       "Plans": [                                 +
         {                                        +
           "Node Type": "Seq Scan",               +
           "Parent Relationship": "Outer",        +
           "Parallel Aware": false,               +
           "Async Capable": false,                +
           "Relation Name": "t_test_d",           +
           "Schema": "public",                    +
           "Alias": "t_test_d",                   +
           "Startup Cost": 0.00,                  +
           "Total Cost": 38.25,                   +
           "Plan Rows": 11,                       +
           "Plan Width": 6,                       +
           "Output": ["t_test_d.ctid"],           +
           "Filter": "(t_test_d.id = '1'::bigint)"+
         }                                        +
       ]                                          +
     }                                            +
   }                                              +
 ]
(1 row)