[Feature] Dynamic Tables

[avamingli]

Description

Snowflake has a special table called Dynamic Tables: https://docs.snowflake.com/en/user-guide/dynamic-tables-intro

The Dynamic Tables is much like Materialized View but in Snowflake,
Materialized View is not exactly the one in Postgres/GPDB/CBDB.
Here is difference between Materialized Views and Dynamic Tables in Snowflake:
https://docs.snowflake.com/en/user-guide/dynamic-tables-comparison#dynamic-tables-compared-to-materialized-views

A main difference is that Materialized View could be used to Rewrite Query in Snowflake, but Dynamic tables can't.
Rewrite Query in Snowflake is our Answer Query Using Materialized Views(AQUMV) in CBDB.

And Materialized View could only has one simple table in Snowflake, that is exactly the matview candidate we could use for AQUMV currently(But we are planning to support complex querys have multiple tables or joins).

Dynamic Tables are very useful for users in some cases.
We don't have exactly one like Materialized View or Dynamic Tables in Snowflake, but we could have sth like that.
Maybe not all concepts or features are exactly matched, but most of them could be implemented in Postgres/CBDB.

And if we have Dynamic Tables in CBDB, unlike Snowflake, I think it could be used to AQUMV too.
Because we have the ability to track matview data's status and even if it has external tables #693 with a GUC to control.

Use case/motivation

No response

Related issues

No response

Are you willing to submit a PR?

• Yes I am willing to submit a PR!

[my-ship-it]

It's an interesting topic.
Dynamic Table is useful in certain situations, especially in scenarios where the requirements for results are not absolutely correct. In addition, refreshing Dynamic Table requires some independent computing resources, which will be more widely used in cloud environments.

[jianlirong]

I think we can leverage the combination of these techniques, including materialized views for external tables, dynamic tables (auto refreshing materialized views) and AQUMV to solve the problem often raised by customers who are big fans of a lakehouse architecture: how can we run queries on external tables as fast as internal tables?

A good example that comes to mind is this: we create a Foreign Table pointing to Apache Hive using FDW, and then create a Dynamic Table based on this Foreign Table that can be refreshed automatically (either periodically based on a time interval or triggered by listening to HMS CDC events). When a user queries the Foreign Table, we use AQUMV technology to replace the Foreign Table with the corresponding Dynamic Table, significantly improving query time.

What's more, we should support setting distribution keys for dynamic tables, while foreign tables are commonly thought of as having a random distribution.

More details need to be figured out, and I think this is a promising direction.

[avamingli]

All agree.
That kind of Dynamic Table will be a powerful and very efficient tool to query remote data for users.

[avamingli]

Another feature of Dynamic Tables in Snowflake is: users could specify the query used to transform the data from one or more base objects or dynamic tables.

https://docs.snowflake.com/en/user-guide/dynamic-tables-about

All of that could be supported, and we could do more: Besides base tables, foreign tables , dynamic tables, Dynamic Tables could be composed from materialized views as well.
Each Dynamic Table has its own auto-refresh process.

[avamingli]

I have been coding some functions, here are design details:

DDL:

Provide a base table

CREATE TABLE t1(a int, b int, c int) distributed by (b);
INSERT INTO t1 SELECT i, i + 1, i + 2 FROM GENERATE_SERIES(1, 10) i;
INSERT INTO t1 SELECT i, i + 1, i + 2 FROM GENERATE_SERIES(1, 5) i;

Create Dynamic Table:

CREATE DYNAMIC TABLE dt0 SCHEDULE '5 * * * *' AS
  SELECT a, b, sum(c) FROM t1 GROUP BY a, b WITH NO DATA DISTRIBUTED BY(b);
CREATE DYNAMIC TABLE

\d
                 List of relations
 Schema | Name |     Type      |  Owner  | Storage
--------+------+---------------+---------+---------
 public | dt0  | dynamic table | gpadmin | heap
 public | t1   | table         | gpadmin | heap
(2 rows)

CREATE DYNAMIC TABLE xxx AS Query
The Query allows any valid SELECT SQL of Materialized Views: from single or multiple relations, base tables, materialized views, and dynamic tables as well, joins, subquery, aggregation, group by and etc.
However, if you want to use it to Answer Query, that is limited by AQUMV: currently we allow Select from single base table, aggregation on it or aggregation SQL replace directly #705

SCHEDULE:

A string used to schedule background job which auto-refreshes the dynamic table.
We follow the valid string of pg_cron extension which supports linux crontab, refer https://crontab.guru/ .

 ┌───────────── min (0 - 59)
 │ ┌────────────── hour (0 - 23)
 │ │ ┌─────────────── day of month (1 - 31) or last day of the month ($)
 │ │ │ ┌──────────────── month (1 - 12)
 │ │ │ │ ┌───────────────── day of week (0 - 6) (0 to 6 are Sunday to
 │ │ │ │ │                  Saturday, or use names; 7 is also Sunday)
 │ │ │ │ │
 │ │ │ │ │
 * * * * *

You can also use '[1-59] seconds' to schedule a job based on an interval.
The example creates a cron job refreshing the dynamic table at minute 5 of each hour.

User don't need to consider the auto-refresh job, however query on pg_task catalog if we want to see the task:

SELECT * FROM pg_task WHERE jobname LIKE 'gp_dynamic_table_refresh%' AND command LIKE '%dt0';
 jobid | schedule  |             command              | nodename  | nodeport | database | username | active |
   jobname
-------+-----------+----------------------------------+-----------+----------+----------+----------+--------+---------
-----------------------
 17398 | 5 * * * * | REFRESH DYNAMIC TABLE public.dt0 | 127.0.0.1 |     9000 | gpadmin  | gpadmin  | t      | gp_dynam
ic_table_refresh_17394
(1 row)

As Snowflake, Dynamic Tables should always have a auto-refresh process.
However, for convenience, I make SCHEDULE optional. If user didn't specific it, a default schedule is provided: maybe at every 5th minute(snowflake limit at most 5 minutes for dynamic table auto-refresh, not sure)?

WITH NO DATA:

Same as Materialized View, will create an empty Dynamic Table if specified.

DISTRIBUTED BY:

Same as normal tables in CBDB, Dynamic Tables could support distribution keys as materialized views.
Use \d+ to see the distribution keys and the Query SQL of Dynamic Tables.

\d+ dt0;
                                       Dynamic table "public.dt0"
 Column |  Type   | Collation | Nullable | Default | Storage | Compression | Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
 a      | integer |           |          |         | plain   |             |              |
 b      | integer |           |          |         | plain   |             |              |
 sum    | bigint  |           |          |         | plain   |             |              |
View definition:
 SELECT t1.a,
    t1.b,
    sum(t1.c) AS sum
   FROM t1
  GROUP BY t1.a, t1.b;
Distributed by: (b)
Access method: heap

Refresh Dynamic Table

As seen in pg_task, we put a command to auto-refresh dynamic tables.
However, if users want to do a REFRESH manually, exec command REFRESH DYNAMIC TABLE is also supported.

REFRESH DYNAMIC TABLE dt0;
REFRESH DYNAMIC TABLE

REFRESH WITH NO DATA;

Same as Materialized Views, Refresh with no data will truncate the Dynamic Table and make it unpopulated status.

REFRESH DYNAMIC TABLE dt0 WITH NO DATA;
REFRESH DYNAMIC TABLE

Drop Dynamic Table:

DROP DYNAMIC TABLE dt0;
DROP DYNAMIC TABLE

Drop a Dynamic Table will drop its scheduler job automatically.

SELECT * FROM pg_task WHERE jobname LIKE 'gp_dynamic_table_refresh%' AND command LIKE '%dt0';
 jobid | schedule | command | nodename | nodeport | database | username | active | jobname
-------+----------+---------+----------+----------+----------+----------+--------+---------
(0 rows)

Privileges

Same as Materialized Views in CBDB:

\z
                                Access privileges
 Schema | Name |     Type      | Access privileges | Column privileges | Policies
--------+------+---------------+-------------------+-------------------+----------
 public | dt1  | dynamic table |                   |                   |
 public | t1   | table         |                   |                   |
(2 rows)

Use Dynamic Tables to answer query

Like Materialized Views, Dynamic Tables could be used to answer query too:

CREATE DYNAMIC TABLE dt1  AS
  SELECT * FROM t1 WHERE a = 1 DISTRIBUTED BY(b);
ANALYZE dt1;
SELECT 2

SET enable_answer_query_using_materialized_views = ON;
EXPLAIN(COSTS OFF, VERBOSE)
SELECT * FROM t1 WHERE a = 1;
SELECT * FROM t1 WHERE a = 1;
SET
                                    QUERY PLAN
----------------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)
   Output: a, b, c
   ->  Seq Scan on public.dt1
         Output: a, b, c
 Settings: enable_answer_query_using_materialized_views = 'on', optimizer = 'off'
 Optimizer: Postgres query optimizer
(6 rows)

 a | b | c
---+---+---
 1 | 2 | 3
 1 | 2 | 3
(2 rows)

I think we can leverage the combination of these techniques, including materialized views for external tables, dynamic tables (auto refreshing materialized views) and AQUMV to solve the problem often raised by customers who are big fans of a lakehouse architecture: how can we run queries on external tables as fast as internal tables?

We will enable that after #702 is merged, with Dynamic Tables feature.

[avamingli]

Another thing is: do we need a ALTER DYNAMIC TABLE command?

It may be used to: SUSPEND/RESUME(Snowflake grammar, pg_cron has ACTIVE/NOT ACTIVE instead) the background auto-refreshing job of Dynamic Table
An active refresh process will block the cluster management, ex: when you want to use gpstop.
If the interval of background auto-refreshing job is very small, and there are lots of those, gpstop may be blocked for a certain time.
We got the similar issue in commercial version.

However, users could manage the task directly using ALTER TASK command from pg_cron, it would be easy if we could provide a function to show the auto-refreshing job of Dynamic Table.

[avamingli]

See codes in PR #725