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This is for two reasons. Firstly, we need to track the types of inputs, their values, and also the input variables; adding a struct gives us a little more clarity. Secondly, when we come to implement prepared statements, we'll be algebrizing queries without having the values available. We'll be able to do a better job of algebrizing, and also do more validating, if we allow callers to specify the types of variables in advance, even if the values aren't known. |
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This crate turns a parsed query, as defined by the query crate and produced by the query-parser crate, into an algebrized tree, also called a query processor tree.
This is something of a wooly definition: a query algebrizer in a traditional relational database is the component that combines the schema — including column type constraints — with the query, resolving names and that sort of thing. Much of that work is unnecessary in our model; for example, we don't need to resolve column aliases, deal with table names, or that sort of thing. But the similarity is strong enough to give us the name of this crate.
The result of this process is traditionally handed to the query optimizer to yield an execution plan. In our case the execution plan is deterministically derived from the algebrized tree, and the real optimization (such as it is) takes place within the underlying SQLite database.