[Import/Export] Exported databases can now be safely moved

[Tishj]

This PR fixes #5868

Previously we would EXPORT and bake the absolute path of the table data into the query e.g path/to/exported/db/tbl.csv
When we only really need the tbl.csv part, because the path of the db folder is already provided on IMPORT.

Now we prefix the path with a placeholder on EXPORT, and replace it with the db folder path on IMPORT.

[Tishj]

Actually, do we even need the placeholder?

[Mytherin]

Thanks for the PR!

While the placeholder approach is relatively straightforward I do see some potential issues with this:

• It won't work correctly with older versions of DuckDB - as they won't know how to handle a placeholder, and won't emit a placeholder
• It won't work when loading into other systems, e.g. Postgres
• Global text replacement is a bit hacky, since the IMPORT file can contain arbitrary user-defined input (e.g. the actual file name)

Perhaps we can use the Parser for this - for example we can run the parser on the load.sql file. It should only contain COPY statements - and we can directly extract and modify the path for each of the CopyStatements within the file. We could then implement CopyStatement::ToString (similar to what we have for SelectStatement) to stringify it again. That should work while leaving the export code untouched.

[Tishj]

You mean from new version -> old version
Is that supported in general?

Because for old version -> new version this should still work
it doesn't assert that the file is prefixed with the placeholder, only when it does it replaces it

The other points are fair though
I like the approach you suggested, I felt the string replacements were kinda dirty/error-prone to begin with

[Mytherin]

You mean from new version -> old version
Is that supported in general?

It used to work, so I would prefer not to break it unnecessarily

[Tishj]

I've made some changes to the statement verifier to include the copy statement

Though I went a little overboard and implemented Equals for every Statement
I think I'll make a separate PR out of it, and also add Copy + Equals verification for all statements

Found and fixed a few bugs in this ToString that I need to move over to this PR, but other than that it should be done

[Mytherin]

We have verification for ToString round-trips for INSERT/DELETE/SELECT statements in src/main/client_context.cpp:670 (in PendingStatementOrPreparedStatementInternal). Perhaps add the COPY statement there as well?

[Tishj]

Oh I completely missed that, my mind instantly went to StatementVerifier
so I made changes there to support other types of statements than Select, but that required skipping a lot of the functionality of the StatementVerifier, because we can't execute them - because of side-effects.

So this will definitely be better
Also a good place to add the Equals check I think

[Mytherin]

Thanks!