Re: [D] [Feature] Dynamic Tables [cloudberry]

["avamingli (via GitHub)" <gi...@apache.org>]

GitHub user avamingli added a comment to the discussion: [Feature] Dynamic Tables

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

## DDL:

Provide a base table 
```sql
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:

```sql
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)
```
#### 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/ .
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:
```sql
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 5-minutes(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.
```sql
\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.
```sql
REFRESH DYNAMIC TABLE dt0;
REFRESH DYNAMIC TABLE
```

#### Drop Dynamic Table:
```sql
DROP DYNAMIC TABLE dt0;
DROP DYNAMIC TABLE
```
Drop a Dynamic Table will drop its scheduler job automatically.
```sql
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:
```sql
\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:
```sql
CREATE DYNAMIC TABLE dt1  AS
  SELECT * FROM t1 WHERE a = 1 DISTRIBUTED BY(b);
ANALYZE dt1;
SELECT 2
```
```sql
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 archive that after #702 is merged, with Dynamic Tables feature.








GitHub link: https://github.com/apache/cloudberry/discussions/706#discussioncomment-11316615

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