[AQUMV]Enable answer query using Materialized View for external table.

[avamingli]

Allow answer query using materialized views which have external or foreign tables. Since we don't know if the data is up to date of externel table outside CBDB, introduce a new GUC:

aqumv_allow_foreign_table

Let user decide if they want to use matview instead of query on external tables.

create readable external table aqumv_ext_r(id int) 
location ('demoprot://aqumvtextfile.txt') format 'text';

create materialized view aqumv_ext_mv as
  select * from aqumv_ext_r;

explain (costs off, verbose)
select * from aqumv_ext_r;
               QUERY PLAN
------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)
   Output: id
   ->  Seq Scan on aqumv.aqumv_ext_mv
         Output: id
 Optimizer: Postgres query optimizer

Index could also be used if there were on matviews.

create index on aqumv_ext_mv(id);
explain (costs off, verbose)
select * from aqumv_ext_r where id = 5;
                            QUERY PLAN
----------------------------------------------------------------------
 Gather Motion 1:1  (slice1; segments: 1)
   Output: id
   ->  Index Only Scan using aqumv_ext_mv_id_idx on aqumv.aqumv_ext_mv
         Output: id
         Index Cond: (aqumv_ext_mv.id = 5)
 Optimizer: Postgres query optimizer

fix #ISSUE_Number

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[avamingli]

see more details in #693

[reshke]

aqumv_allow_foreign_table

PostgreSQL-style GUC would have name like enable_XXX, huh? So, maybe enable_aqumv_foreign_table

[avamingli]

aqumv_allow_foreign_table

PostgreSQL-style GUC would have name like enable_XXX, huh? So, maybe enable_aqumv_foreign_table

Not sure.. I follow this one: allow_system_table_mods

[avamingli]

@my-ship-it As we have refresh fast path at #682, but for external tables we don't know the status(always up to date in gp_maview_aux). This will make REFRESH command fail to do the real thing from external data.

So, we should skip fast path for the views have external tables. That need catalog change to record if a view has external tables outside CBDB.

[my-ship-it]

@my-ship-it As we have refresh fast path at #682, but for external tables we don't know the status(always up to date in gp_maview_aux). This will make REFRESH command fail to do the real thing from external data.

So, we should skip fast path for the views have external tables. That need catalog change to record if a view has external tables outside CBDB.

Yes, I think it is a reasonable behavior. Thanks!