doc internal

ARCHITECTURE.md

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+## `vec0`
+
+### idxStr
+
+The `vec0` idxStr is a string composed of single "header" character and 0 or
+more "blocks" of 4 characters each.
+
+The "header" charcter denotes the type of query plan, as determined by the
+`enum vec0_query_plan` values. The current possible values are:
+
+| Name                       | Value | Description                                                            |
+| -------------------------- | ----- | ---------------------------------------------------------------------- |
+| `VEC0_QUERY_PLAN_FULLSCAN` | `'1'` | Perform a full-scan on all rows                                        |
+| `VEC0_QUERY_PLAN_POINT`    | `'2'` | Perform a single-lookup point query for the provided rowid             |
+| `VEC0_QUERY_PLAN_KNN`      | `'3'` | Perform a KNN-style query on the provided query vector and parameters. |
+
+Each 4-character "block" is associated with a corresponding value in `argv[]`. For example, the 1st block at byte offset `1-4` (inclusive) is the 1st block and is associated with `argv[1]`. The 2nd block at byte offset `5-8` (inclusive) is associated with `argv[2]` and so on. Each block describes what kind of value or filter the given `argv[i]` value is.
+
+
+#### `VEC0_IDXSTR_KIND_KNN_MATCH` (`'{'`)
+
+`argv[i]` is the query vector of the KNN query.
+
+The remaining 3 characters of the block are `_` fillers.
+
+#### `VEC0_IDXSTR_KIND_KNN_K` (`'}'`)
+
+`argv[i]` is the limit/k value of the KNN query.
+
+The remaining 3 characters of the block are `_` fillers.
+
+#### `VEC0_IDXSTR_KIND_KNN_ROWID_IN` (`'['`)
+
+`argv[i]` is the optional `rowid in (...)` value, and must be handled with [`sqlite3_vtab_in_first()` /
+`sqlite3_vtab_in_next()`](https://www.sqlite.org/c3ref/vtab_in_first.html).
+
+The remaining 3 characters of the block are `_` fillers.
+
+#### `VEC0_IDXSTR_KIND_KNN_PARTITON_CONSTRAINT` (`']'`)
+
+`argv[i]` is a "constraint" on a specific partition key.
+
+The second character of the block denotes which partition key to filter on, using `A` to denote the first partition key column, `B` for the second, etc. It is encoded with `'A' + partition_idx` and can be decoded with `c - 'A'`.
+
+The third character of the block denotes which operator is used in the constraint. It will be one of the values of `enum vec0_partition_operator`, as only a subset of operations are supported on partition keys.
+
+The fourth character of the block is a `_` filler.
+
+
+#### `VEC0_IDXSTR_KIND_POINT_ID` (`'!'`)
+
+`argv[i]` is the value of the rowid or id to match against for the point query.
+
+The remaining 3 characters of the block are `_` fillers.