diff --git a/src/lib/mina_caqti/README.md b/src/lib/mina_caqti/README.md
new file mode 100644
index 00000000000..ed3beb29610
--- /dev/null
+++ b/src/lib/mina_caqti/README.md
@@ -0,0 +1,77 @@
+Mina_caqti
+==========
+
+This library is designed to assist in querying relational databases
+using the Caqti library. It is used extensively for querying the
+archive database in the `Processor` and `Load_data` modules in
+`Archive_lib`.
+
+Constructing SQL queries
+------------------------
+
+Instead of writing out SQL queries as text, the
+functions here can construct those queries from table information.
+
+For example, the `Token` module in the archive processor contains:
+```ocaml
+  let table_name = "tokens"
+
+  let find_by_id (module Conn : CONNECTION) id =
+    Conn.find
+      (Caqti_request.find Caqti_type.int typ
+         (Mina_caqti.select_cols_from_id ~table_name ~cols:Fields.names) )
+      id
+```
+The list `Fields.names` is generated from the `deriving fields` annotation on
+the type `Token.t`. The call to `select_cols_fromid` constructs the query
+```
+SELECT value,owner_public_key_id,owner_token_id FROM tokens WHERE id = ?
+```
+
+There are other SQL-building functions in the library, like
+`select_cols`, `insert_into_cols`, and `select_insert_into_cols`, which
+are documented in the source code.
+
+Custom array types
+------------------
+
+You can use custom array types to provide a `Caqti.Type.t` for OCaml array types not
+already built into Caqti.  For example, `array_int_typ` is used to
+give a type for the OCaml type `int array`. Such Caqti types can be
+used for the input or result type of queries, or to provide type
+annotations on columns in queries. In some cases, PostgreSQL may not
+be able to decode data without such annotations. There's an example of
+using an annotation in
+`Archive_lib.Processor.Zkapp_field_array.add_if_doesn't_exist`.
+
+Encoding values as NULLs
+------------------------
+
+In the descriptions of the functions that follow, please note that the
+values returned are in the `Deferred` monad, because they are the
+result of database queries.
+- For the `add...` functions, the result
+actually has a `Deferred.Result.t` type because queries can fail.
+- For
+the `get...` functions, a failure raises an exception.
+
+There are some zkApps-related functions that are useful for storing
+`Set_or_keep.t` and `Or_ignore.t` values. The function
+`add_if_zkapp_set` runs a query if the data is `Set`, returning its
+result (if it succeeds), and if the data is `Keep`, returns `None`.
+Similarly, `add_if_zkapp_check` runs a query if the data is `Check`,
+returning its result (if it succeeds), and if the data is `Ignore`,
+returns `None`.  The functions `get_zkapp_set_or_keep` and
+`get_zkapp_or_ignore` operate symmetrically, by converting a queried value to
+a value construct with `Set` or `Check`, if not NULL, and converting a
+NULL to `Keep` or `Ignore`. The use of NULL to encode these
+zkApp-related values is mentioned as the `NULL convention` in the part
+of the database schema in `zkapp_tables.sql`.
+
+The functions `add_if_some` and `get_opt_item` are similar to these
+zkApps-related functions, except that the constructors involved are
+`Some` and `None` for option types. Therefore, `add_if_some` runs its
+query argument if the data has `Some` as its constructor, returning
+the result, and otherwise returns `None`. The function `get_opt_item`
+returns a `Some`-constructed value, if the item is not NULL in the
+database, and `None` otherwise.
diff --git a/src/lib/mina_caqti/mina_caqti.ml b/src/lib/mina_caqti/mina_caqti.ml
index 8377968c98f..151b01e9056 100644
--- a/src/lib/mina_caqti/mina_caqti.ml
+++ b/src/lib/mina_caqti/mina_caqti.ml
@@ -5,6 +5,7 @@ open Core_kernel
 open Caqti_async
 open Mina_base
 
+(* custom Caqti types for generating type annotations on queries *)
 type _ Caqti_type.field +=
   | Array_nullable_int : int option array Caqti_type.field
 
@@ -225,10 +226,12 @@ let add_if_some (f : 'arg -> ('res, 'err) Deferred.Result.t) :
     'arg option -> ('res option, 'err) Deferred.Result.t =
   Fn.compose deferred_result_lift_opt @@ Option.map ~f
 
+(* if zkApp-related item is Set, run `f` *)
 let add_if_zkapp_set (f : 'arg -> ('res, 'err) Deferred.Result.t) :
     'arg Zkapp_basic.Set_or_keep.t -> ('res option, 'err) Deferred.Result.t =
   Fn.compose (add_if_some f) Zkapp_basic.Set_or_keep.to_option
 
+(* if zkApp-related item is Check, run `f` *)
 let add_if_zkapp_check (f : 'arg -> ('res, 'err) Deferred.Result.t) :
     'arg Zkapp_basic.Or_ignore.t -> ('res option, 'err) Deferred.Result.t =
   Fn.compose (add_if_some f) Zkapp_basic.Or_ignore.to_option
@@ -278,6 +281,9 @@ let insert_into_cols ~(returning : string) ~(table_name : string)
     (String.concat ~sep:", " cols)
     values returning
 
+(* run `select_cols` and return the result, if found
+   if not found, run `insert_into_cols` and return the result
+*)
 let select_insert_into_cols ~(select : string * 'select Caqti_type.t)
     ~(table_name : string) ?tannot ~(cols : string list * 'cols Caqti_type.t)
     (module Conn : CONNECTION) (value : 'cols) =
@@ -318,11 +324,13 @@ let make_get_opt ~of_option ~f item_opt =
   in
   of_option res_opt
 
+(** convert options to Set or Keep for zkApps-related results *)
 let get_zkapp_set_or_keep (item_opt : 'arg option)
     ~(f : 'arg -> ('res, _) Deferred.Result.t) :
     'res Zkapp_basic.Set_or_keep.t Deferred.t =
   make_get_opt ~of_option:Zkapp_basic.Set_or_keep.of_option ~f item_opt
 
+(** convert options to Check or Ignore for zkApps-related results *)
 let get_zkapp_or_ignore (item_opt : 'arg option)
     ~(f : 'arg -> ('res, _) Deferred.Result.t) :
     'res Zkapp_basic.Or_ignore.t Deferred.t =