Moving updated-at timestamps to unique millisecond times (#17489)

To allow the ability to have guaranteed-unique cursors, we make two
important updates:
* Add new updated_at_ms and created_at_ms columns that are in
millisecond precision
* Guarantee uniqueness -- if multiple items are inserted at the same
millisecond, bump the new one by one millisecond until it becomes unique

This lets us use single-number cursors for forwards and backwards paging
through resultsets and guarantee that the cursor is a fixed point to do
(timestamp > cursor) and get new items only.

This updated implementation is backwards-compatible since multiple
appservers can be running and won't handle the previous method well.

codex-rs/state/migrations/0025_thread_timestamps_millis.sql

@@ -0,0 +1,112 @@
+ALTER TABLE threads ADD COLUMN created_at_ms INTEGER;
+ALTER TABLE threads ADD COLUMN updated_at_ms INTEGER;
+
+CREATE TEMP TABLE thread_timestamp_migration AS
+SELECT
+    id,
+    CASE
+        WHEN created_at < 1577836800000 THEN created_at * 1000
+        ELSE created_at
+    END AS created_at_base_ms,
+    CASE
+        WHEN updated_at < 1577836800000 THEN updated_at * 1000
+        ELSE updated_at
+    END AS updated_at_base_ms
+FROM threads;
+
+WITH RECURSIVE
+ordered_created AS (
+    SELECT
+        id,
+        created_at_base_ms,
+        ROW_NUMBER() OVER (ORDER BY created_at_base_ms, id) AS row_number
+    FROM thread_timestamp_migration
+),
+assigned_created(row_number, id, created_at_ms) AS (
+    SELECT row_number, id, created_at_base_ms
+    FROM ordered_created
+    WHERE row_number = 1
+    UNION ALL
+    SELECT
+        ordered_created.row_number,
+        ordered_created.id,
+        MAX(ordered_created.created_at_base_ms, assigned_created.created_at_ms + 1)
+    FROM ordered_created
+    JOIN assigned_created ON ordered_created.row_number = assigned_created.row_number + 1
+)
+UPDATE threads
+SET created_at_ms = (
+    SELECT created_at_ms
+    FROM assigned_created
+    WHERE assigned_created.id = threads.id
+);
+
+WITH RECURSIVE
+ordered_updated AS (
+    SELECT
+        id,
+        updated_at_base_ms,
+        ROW_NUMBER() OVER (ORDER BY updated_at_base_ms, id) AS row_number
+    FROM thread_timestamp_migration
+),
+assigned_updated(row_number, id, updated_at_ms) AS (
+    SELECT row_number, id, updated_at_base_ms
+    FROM ordered_updated
+    WHERE row_number = 1
+    UNION ALL
+    SELECT
+        ordered_updated.row_number,
+        ordered_updated.id,
+        MAX(ordered_updated.updated_at_base_ms, assigned_updated.updated_at_ms + 1)
+    FROM ordered_updated
+    JOIN assigned_updated ON ordered_updated.row_number = assigned_updated.row_number + 1
+)
+UPDATE threads
+SET updated_at_ms = (
+    SELECT updated_at_ms
+    FROM assigned_updated
+    WHERE assigned_updated.id = threads.id
+);
+
+DROP TABLE thread_timestamp_migration;
+
+CREATE TRIGGER threads_created_at_ms_after_insert
+AFTER INSERT ON threads
+WHEN NEW.created_at_ms IS NULL
+BEGIN
+    UPDATE threads
+    SET created_at_ms = NEW.created_at * 1000
+    WHERE id = NEW.id;
+END;
+
+CREATE TRIGGER threads_updated_at_ms_after_insert
+AFTER INSERT ON threads
+WHEN NEW.updated_at_ms IS NULL
+BEGIN
+    UPDATE threads
+    SET updated_at_ms = NEW.updated_at * 1000
+    WHERE id = NEW.id;
+END;
+
+CREATE TRIGGER threads_created_at_ms_after_update
+AFTER UPDATE OF created_at ON threads
+WHEN NEW.created_at != OLD.created_at
+ AND NEW.created_at_ms IS OLD.created_at_ms
+BEGIN
+    UPDATE threads
+    SET created_at_ms = NEW.created_at * 1000
+    WHERE id = NEW.id;
+END;
+
+CREATE TRIGGER threads_updated_at_ms_after_update
+AFTER UPDATE OF updated_at ON threads
+WHEN NEW.updated_at != OLD.updated_at
+ AND NEW.updated_at_ms IS OLD.updated_at_ms
+BEGIN
+    UPDATE threads
+    SET updated_at_ms = NEW.updated_at * 1000
+    WHERE id = NEW.id;
+END;
+
+CREATE INDEX idx_threads_created_at_ms ON threads(created_at_ms DESC, id DESC);
+CREATE INDEX idx_threads_updated_at_ms ON threads(updated_at_ms DESC, id DESC);