Add a more interesting README

README.md

@@ -3,7 +3,7 @@
 
 PG Raw Parse provides a low level wrapper around the PostgreSQL backend parser.
 These bindings, as well as some additional functionality are provided by
-[libpg\_query](https://github.com/pganalyze/libpg_query).
+[libpg\_query].
 
 In addition to parsing, we provide mechanisms to [traverse an AST], [construct
 new ASTs], and [transform ASTs]. See the API docs for more details.
@@ -12,12 +12,136 @@ new ASTs], and [transform ASTs]. See the API docs for more details.
 [construct new ASTs]: https://docs.rs/pg_raw_parse/latest/pg_raw_parse/make/index.html
 [transform ASTs]: https://docs.rs/pg_raw_parse/latest/pg_raw_parse/transform/index.html
 
+## Design
+
+The primary goal of `pg_raw_parse` is to map to PostgreSQL's parser with as
+little overhead as possible. This means mapping to the raw structures whenever
+possible, using PostgreSQL's internal allocator, and avoiding any significant
+copies of data.
+
+PostgreSQL does not publish any header files or libraries to expose its backend
+functions. We use [libpg\_query], which embeds those files in a form that is
+easy to compile without going through cmake, as well as makes a few changes
+to enable multithreaded usage. We also use this library for its `deparse`
+implementation, turning an AST back into a string.
+
+When possible, the structures in this library are cast directly from a pointer
+to the C structure. The main exception to this is `Node *`, which is
+semantically an unsized enum. There is no way to represent an enum with
+different sizes for each variant in Rust, so we need our own wrapper enum. The
+tag is identical to the tag of the C enum, so LLVM *should* be able to optimize
+this away in many cases but it is not guaranteed.
+
+Everything in pg\_raw\_parse makes use of PostgreSQLs allocator, both for
+manipulating the structures returned by `parse`, and for [constructors provided
+by this library][construct new ASTs]. It is assumed that ASTs are retained at
+the scope of a single query. Each call to `parse` will return an AST with its
+own arena. Individual nodes do not implement `Drop`, and are not freed until the
+entire arena is dropped. This can result in slightly higher memory usage when
+mutating ASTs, as nodes that are replaced will still occupy memory. But the
+result is much less overhead from `palloc`/`pfree` in the most common usage
+patterns.
+
+To ensure that fields of an AST node are always allocated on the same arena as
+its parent, we make use of [lifetime branding]. [`MemoryToken`] is a type that
+is used for constructing node allocated on a specific arena. Constructors
+require all fields to be [`Unique`], which represents a node allocated on that
+same arena and is not assigned anywhere else. Once all construction/mutation is
+complete, the result is wrapped in [`Owned`], which is responsible for freeing
+the arena in its destructor.
+
+[lifetime branding]: https://plv.mpi-sws.org/rustbelt/ghostcell/
+[`MemoryToken`]: https://docs.rs/pg_raw_parse/latest/pg_raw_parse/make/struct.MemoryToken.html
+[`Unique`]: https://docs.rs/pg_raw_parse/latest/pg_raw_parse/make/struct.Unique.html
+[`Owned`]: https://docs.rs/pg_raw_parse/latest/pg_raw_parse/struct.Owned.html
+
+Because individual nodes are never freed on their own, once an arena is inside
+of an `Owned`, it is frozen. It is only possible to get shared references to
+fields within it, and its arena can never be used for allocations again. This
+decision was made to make it impossible to cause a memory leak by holding a long
+lived reference to an AST, and then mutating it repeatedly. Instead, to mutate
+an `Owned` node, it must first be copied onto a new memory arena using
+[`make_unique`].
+
+[`make_unique`]: https://docs.rs/pg_raw_parse/latest/pg_raw_parse/make/struct.MemoryToken.html#method.make_unique
+
+The majority of the code in this library is generated from C header files, with
+the exception of extremely generic code such as list manipulation. We first run
+these header files through [bindgen], and then operate on the resulting code as
+if it were a procedural macro. Although this code lives in
+[build.rs](blob/main/build.rs), its patterns should be familiar to developers
+familiar with writing procedural macros.
+
+[bindgen]: https://github.com/rust-lang/rust-bindgen
+
+We create our own layout compatible structs rather than directly exposing the
+structs generated by bindgen. This is to give us control over the visibility of
+fields, as we don't want raw pointer fields to be public. We generate accessor
+methods that convert to our custom type, and check the tag so an invalid node
+assignment results in a panic rather than undefined behavior. In particular,
+this is required for `Node*`, which cannot be represented in Rust as a simple
+pointer cast for the reasons mentioned above.
+
+C has no concept of generics, so all lists are untyped lists of nodes. However,
+many of those fields have documentation stating that they are a list of a single
+type of node. We look for those comments, and change the type of the field to a
+typed list if we find one.
+
+[AST traversal][walk an AST] is done using PostgreSQL's internal
+`raw_expression_tree_walker` function, with a thin wrapper to handle passing a
+Rust closure to C and transform PostgreSQL's exceptions into Rust panics. [AST
+transformation][transform ASTs] is done with generated code.
+
+As a result of relying on code generation for the majority of this library,
+supporting new PostgreSQL versions requires very little work. It is usually
+nothing more than a submodule update for [libpg\_query], pointing to a commit
+which includes the PostgreSQL source for that version.
+
+## Comparison with pg_query.rs
+
+The other popular library in this space is [pg\_query.rs], which is maintained
+by the same team who maintains [libpg\_query]. While both libraries depend on
+[libpg\_query] to get access to PostgreSQL's internal parser, [pg\_query.rs]
+uses [libpg\_query]'s protobuf serialization layer to somewhat decouple it from
+PostgreSQL's internal details. This type of approach makes sense when you're
+maintaining bindings for multiple languages. But Rust's strong C FFI means a
+lower level binding allows us to avoid many of the drawbacks of that approach.
+
+We are able to avoid the overhead of protobuf de/serialization, as well as
+memory cost of copying all those structures into a memory space controlled by
+the global allocator. `protoc` also generates a fairly inefficient data
+structure in this case, causing the `Node` enum to be 584 bytes large.
+
+In contrast, by binding directly to PostgreSQL's data structures, there is no
+memory overhead beyond what would be used either way within PostgreSQL's parser.
+And the cost of "constructing" the Rust structures is at most a pointer cast and
+a tag check. These two factors result in pg\_raw\_parse performing significantly
+better, with the gap increasing as the size of the AST increases.
+
+#### Parse time
+
+![Speed benchmark graph](raw/main/benchmark_time.png)
+
+#### Parse time (log scale)
+
+![Speed benchmark graph (log scale)](raw/main/benchmark_time_log.png)
+
+#### Memory Usage
+
+![Memory benchmark graph](raw/main/benchmark_mem.png)
+
+#### Memory Usage (log scale)
+
+![Memory benchmark graph (log scale)](raw/main/benchmark_mem_log.png)
+
+## Contributing
+
 This library's API surface is primarily driven by the needs of
 [PgDog](https://github.com/pgdogdev/pgdog). It is not intended to be a complete,
 one-size-fits-all solution to PostgreSQL ASTs.  Contributions are welcome, but
 pull requests adding large and complex features are unlikely to be accepted
 unless they align with PgDog's needs. For a more general purpose library,
-consider [pg\_query.rs](https://github.com/pganalyze/pg_query.rs).
+consider [pg\_query.rs].
 
 ## License
 
@@ -27,3 +151,19 @@ Licensed under either of these:
    https://www.apache.org/licenses/LICENSE-2.0)
  * MIT license ([LICENSE-MIT](LICENSE-MIT) or
    https://opensource.org/licenses/MIT)
+
+[libpg\_query]: https://github.com/pganalyze/libpg_query
+[pg\_query.rs]: https://github.com/pganalyze/pg_query.rs
+
+## LLM Policy
+
+This library was primarily written by humans, without LLM assistance. While LLM
+assisted pull requests are allowed, any code written by an LLM must be
+disclosed. Regardless of whether they were written by a human or an LLM, pull
+requests are expected to be of a reasonable size to be reviewed by a human.
+
+Your pull request description, and any communication with maintainers must be
+written by a human. Do not copy/paste output from an LLM into a PR comment. Pull
+requests that violate this policy will be closed and locked.
+
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