Update docs for transaction handling - snowflake.md

[sspaeti]

Description

If you use Snowflake and have transaction settings not set to default, e.g., have set ALTER SESSION SET AUTOCOMMIT = FALSE;, dlt will not work properly and end up having an open session with uncommitted Inserts/merge statements.

I added a note to the docs with this PR to make that clear and not run into errors I did.

Additional Context

Even better would be to explicitly set ALTER SESSION SET AUTOCOMMIT = TRUE; on the insert to destination schema as well as you do for inserting into Staging, where you added ALTER SESSION SET AUTOCOMMIT = TRUE; explicitly.

PS: I added the note to Additional connection options, not sure if that is the best place. Feel free to reorder or let me know where it would make more sense.

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[sspaeti]

Just FYI: I'm not sure if the :::note is not working, or the preview (above link isn't correct), but it does not show my additional note on the preview.

[rudolfix]

@sspaeti how did you manage to change session settings? each loader job gets a separate connection which is closed after COPY/MERGE command. I'm asking because indeed we didn't start the transaction before SQL job and I changed that: we do it in LoadJob managed run method. MERGE/COPY SQL scripts are not affected.

[sspaeti]

@rudolfix It was set at a account level (globally). As it can be set on three levels:

• Account (global) -> alter account set autocommit = false;
• User -> alter user <username> set autocommit = false;
• Session -> alter session set autocommit = false;

Whereas session user and account settings overrule. See https://docs.snowflake.com/en/sql-reference/parameters#label-autocommit.

I believe this happens, because you only explicitly set the ALTER SESSION SET AUTOCOMMIT = TRUE; but not for FALSE. Meaning if someone has it the global/account settings set to FALSE, that's when we ran into the issue of open sessions. Small issues, but might be worth mentioning or make it explicit in both way? 🤔

[rudolfix]

@sspaeti now I get it? should we just set autocommit explicitly to TRUE on opening the connection? we do that anyway after comitting/rollbacking the transaction. if we do that, we just need to mention that we set it explicitly in the docs and user can still have those account settings, right?

[rudolfix]

this PR will explicitly set autocommit on new connection https://github.com/dlt-hub/dlt/pull/2593