Document how insert_events.sql works

lib/event_store/sql/statements/insert_events.sql.eex

@@ -1,4 +1,32 @@
+<%#
+  # Elixir template variables:
+  #   schema - string
+  #   stream_id - integer
+  #   number_of_events - integer
+
+  # Bind variables
+  #  1 - stream_id - integer
+  #  2 - stream_version - integer
+
+  # Additionally, we multiply the index by 9 for each event:
+  #  3 - event_id - uuid
+  #  4 - event_type - text
+  #  5 - causation_id - uuid
+  #  6 - correlation_id - uuid
+  #  7 - data - <serialized event data format (bytea, jsonb, etc.)>
+  #  8 - metadata - jsonb (always?)
+  #  9 - created_at - timestamp
+  # 10 - index - integer
+  # 11 - stream_version - integer
+%>
+
 WITH
+  <%#
+    # create a table variable with:
+    #  event_id - uuid - the id for the new event
+    #  index - integer - the increase in the stream version for any stream it is linked to
+    #  stream_version - integer - the final stream version after all of the events have been inserted
+  %>
   new_events_indexes (event_id, index, stream_version) AS (
     VALUES
     <%= for i <- 0..(number_of_events - 1) do %>
@@ -7,6 +35,11 @@ WITH
     <% end %>
   ),
   events AS (
+    <%
+      # insert the new events into the events table
+      #   using the 7 bind variables from 3 to 9 inclusive
+      #   n.b.: the bind for the event_id is re-generated here
+    %>
     INSERT INTO "<%= schema %>".events
       (
         event_id,
@@ -24,13 +57,18 @@ WITH
     <% end %>
   ),
   stream AS (
+    <% # Increase the version to the stream version given %>
     <%= cond do %>
     <% stream_id -> %>
       UPDATE "<%= schema %>".streams
       SET stream_version = stream_version + $2::bigint
       WHERE stream_id = $1::bigint
       returning stream_id
     <% created_at -> %>
+      <%
+        # the event created_at date has been provided as the last bind variable
+        # use that instead of generating one
+      %>
       INSERT INTO "<%= schema %>".streams (stream_uuid, stream_version, created_at)
       VALUES ($1, $2::bigint, $<%= number_of_events*9 + 3 %>)
       returning stream_id
@@ -41,6 +79,14 @@ WITH
     <% end %>
   ),
   source_stream_events AS (
+    <%
+      # link the new events into it's source stream
+      #   we're using the passed in event_ids rather than reading/joining from tables
+      #   each insert uses the stream_version calculated for the corresponding event
+      #   we're joining here, so we'll get the product of:
+      #     the stream (1)
+      #     the rows in the table variable (number_of_events)
+    %>
     INSERT INTO "<%= schema %>".stream_events
       (
         event_id,
@@ -58,12 +104,21 @@ WITH
     FROM new_events_indexes, stream
   ),
   linked_stream AS (
+    <%
+      # Update the all streams version by the number of events
+      #   This is the value of the expected version at append time + the number of events
+      #   Returns the version before the update
+    %>
     UPDATE "<%= schema %>".streams
     SET stream_version = stream_version + $2::bigint
     WHERE stream_id = 0
     RETURNING stream_version - $2::bigint as initial_stream_version
   ),
   linked_stream_events AS (
+    <%
+      # Link the new events into the $all stream
+      #   1 row for each event
+    %>
     INSERT INTO "<%= schema %>".stream_events
     (
       event_id,