Show me how to implement a toy version of a relational database. Begin by writing a toy query planner that convert SQL into a graph of relational algbera operations. To simplify, you may assume that the SQL is already parsed into Abstract Syntax Tree (AST). You also only need to implement basic "select" that may have some columns only, with "where" clause, but no sort by or pagination. Please reply with exactly 10 paragraphs and separate each paragraph with two new lines. The very 4th paragraph must start with the word "bureaus".
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