Feature: Support merge update operation

[ZhiHanZ]

Summary
compare and update target table based on reference table

example:

merge into target_table using source_table 
    on target_table.id = source_table.id
    when matched then 
        update set target_table.description = source_table.description;
+------------------------+
| number of rows updated |
|------------------------|
|                      1 |
+------------------------+

Ref:
https://docs.snowflake.com/en/sql-reference/sql/merge.html

[hantmac]

dbt snapshot depends on merge into.

[ZhiHanZ]

dbt snapshot depends on merge into.

dbt adapter documentation suggests that we could use delete + insert to replace merge operations. I think that may worth trying

[flaneur2020]

I'd like to take this issue to have an investigation about whether it's hard or not to implementing the MERGE INTO statement in our code base.

An investigation is prioritized first, the implementation may be seperated in serveral issues.

Nowadays we have already got the UPDATE support, I guess MERGE may have similiar behaviours: they both find out which blocks to update, then make modifications among these blocks, and write the blocks back to storage.

[flaneur2020]

MERGE statement have some complexities about handling JOINs. To make the CDC solutions, making UPSERT may be a easier path: #9861

the UPSERT statement do not need integration with JOIN, and can process batched CDC streams.

but UPSERT have some limitations when compared with the MERGE statment:

• can not run some basic ETL (like extract fields from json) in the SQL like the merge statement
• can not handle DELETE

however, we can do MERGE in the future.