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Compile all AST elements always via dialects, never directly #713
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added 4 commits
September 28, 2023 16:37
The root authority on how to do the SQL syntax properly is the dialect — not the AST element itself. AST tree must only carry the intentions of what we want to execute, but not how it should/could be executed. This makes the AST truly independent of dialects and databases, allowing us to: - Focus on the main logic regardless of the SQL capabilities. - Create custom database connectors without involving AST changes every time. Before the change, adding a new database connector with unusual syntax would often require changing the AST elements to direct to `compiler.dialect.some_method()` — with only a subset of SQL being described in dialects. The other rather arbitrary part of SQL syntax was hard-coded in AST elements and could not be easily overridden without such changes. After the change, all the SQL logic is concentrated in one hierarchy of dialects, mostly in one base class.
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dlawin
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Sep 28, 2023
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This PR inverses the flow of AST-to-SQL compilation.
Previously, it was going from the root code -> database -> compiler -> AST nodes, which in turn sometimes (but not always) redirected back to the compiler -> dialect for some pieces of SQL.
As a result, the SQL logic was spread over the code base and was difficult to override per database.
Now, the root code goes -> database -> compiler -> dialect directly. The dialect knows how to render all SQL. Specifically, the base dialect provides the core SQL for all databases, with each and every AST node being possible to override. The AST nodes know nothing about SQL, they only express our intentions of what should be executed, not how.
A very simple chart:
