Take a query that comes out of pg_stat_statements :

SELECT date_trunc ( ? , o . created_at ) AS week , count ( * ) AS total FROM orders o INNER JOIN customers c ON c . id = o . customer_id WHERE o . created_at >= ? AND o . amount > ? AND c . plan = ? GROUP BY ? ORDER BY 2 DESC LIMIT ?

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Six question marks. Each one means something completely different.

The first, inside date_trunc , expects a string like 'week' — it's telling the function which time bucket to use. The second is a timestamp comparing against created_at . The third is a number comparing against amount . The fourth is a string joined through to the customers table — it has to match a plan value over there. The fifth, sitting bare inside GROUP BY , is a positional integer like 1 , pointing back at the first column in the SELECT list. It's not a value, it's an index .…