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  • simonw · 7 ✖

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  • Support cross-database joins · 7 ✖

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781670827 https://github.com/simonw/datasette/issues/283#issuecomment-781670827 https://api.github.com/repos/simonw/datasette/issues/283 MDEyOklzc3VlQ29tbWVudDc4MTY3MDgyNw== simonw 9599 2021-02-18T22:16:46Z 2021-02-18T22:16:46Z OWNER

Demo is now live here: https://latest.datasette.io/_memory

The documentation is at https://docs.datasette.io/en/latest/sql_queries.html#cross-database-queries - it links to this example query: https://latest.datasette.io/_memory?sql=select%0D%0A++%27fixtures%27+as+database%2C+%0D%0Afrom%0D%0A++%5Bfixtures%5D.sqlite_master%0D%0Aunion%0D%0Aselect%0D%0A++%27extra_database%27+as+database%2C+%0D%0Afrom%0D%0A++%5Bextra_database%5D.sqlite_master

 
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 Support cross-database joins 325958506  
781665560 https://github.com/simonw/datasette/issues/283#issuecomment-781665560 https://api.github.com/repos/simonw/datasette/issues/283 MDEyOklzc3VlQ29tbWVudDc4MTY2NTU2MA== simonw 9599 2021-02-18T22:06:14Z 2021-02-18T22:06:14Z OWNER

The implementation in #1232 is ready to land. It's the simplest-thing-that-could-possibly-work: you can run datasette one.db two.db three.db --crossdb and then use the /_memory page to run joins across tables from multiple databases.

It only works on the first 10 databases that were passed to the command-line. This means that if you have a Datasette instance with hundreds of attached databases (see Datasette Library) this won't be particularly useful for you.

So... a better, future version of this feature would be one that lets you join across databases on command - maybe by hitting /_memory?attach=db1&attach=db2 to get a special connection.

Also worth noting: plugins that implement the prepare_connection() hook can attach additional databases - so if you need better, customized support for this one way to handle that would be with a custom plugin.

 
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 Support cross-database joins 325958506  
781593169 https://github.com/simonw/datasette/issues/283#issuecomment-781593169 https://api.github.com/repos/simonw/datasette/issues/283 MDEyOklzc3VlQ29tbWVudDc4MTU5MzE2OQ== simonw 9599 2021-02-18T19:47:34Z 2021-02-18T19:47:34Z OWNER

I have a working version now, moving development to a pull request.

 
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 Support cross-database joins 325958506  
781591015 https://github.com/simonw/datasette/issues/283#issuecomment-781591015 https://api.github.com/repos/simonw/datasette/issues/283 MDEyOklzc3VlQ29tbWVudDc4MTU5MTAxNQ== simonw 9599 2021-02-18T19:44:02Z 2021-02-18T19:44:02Z OWNER

For the moment I'm going to hard-code a SQLITE_LIMIT_ATTACHED=10 constant and only attach the first 10 databases to the _memory connection.

 
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 Support cross-database joins 325958506  
781574786 https://github.com/simonw/datasette/issues/283#issuecomment-781574786 https://api.github.com/repos/simonw/datasette/issues/283 MDEyOklzc3VlQ29tbWVudDc4MTU3NDc4Ng== simonw 9599 2021-02-18T19:15:37Z 2021-02-18T19:15:37Z OWNER

select * from pragma_database_list(); is useful - shows all attached databases for the current connection.

 
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 Support cross-database joins 325958506  
781573676 https://github.com/simonw/datasette/issues/283#issuecomment-781573676 https://api.github.com/repos/simonw/datasette/issues/283 MDEyOklzc3VlQ29tbWVudDc4MTU3MzY3Ng== simonw 9599 2021-02-18T19:13:30Z 2021-02-18T19:13:30Z OWNER

It turns out SQLite defaults to a maximum of 10 attached databases. This can be increased using a compile-time constant, but even with that it cannot be more than 62: https://stackoverflow.com/questions/9845448/attach-limit-10

 
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 Support cross-database joins 325958506  
781077127 https://github.com/simonw/datasette/issues/283#issuecomment-781077127 https://api.github.com/repos/simonw/datasette/issues/283 MDEyOklzc3VlQ29tbWVudDc4MTA3NzEyNw== simonw 9599 2021-02-18T05:56:30Z 2021-02-18T05:57:34Z OWNER

I'm going to to try prototyping the --crossdb option that causes /_memory to connect to all databases as a starting point and see how well that works.

 
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 Support cross-database joins 325958506  

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