--crossdb option for joining across databases (#1232)

* Test for cross-database join, refs #283
* Warn if --crossdb used with more than 10 DBs, refs #283
* latest.datasette.io demo of --crossdb joins, refs #283
* Show attached databases on /_memory page, refs #283
* Documentation for cross-database queries, refs #283

datasette/database.py

@@ -1,4 +1,5 @@
 import asyncio
+from collections import namedtuple
 from pathlib import Path
 import janus
 import queue
@@ -22,6 +23,8 @@ from .inspect import inspect_hash
 
 connections = threading.local()
 
+AttachedDatabase = namedtuple("AttachedDatabase", ("seq", "name", "file"))
+
 
 class Database:
     def __init__(
@@ -78,7 +81,7 @@ class Database:
                 conn.execute("PRAGMA query_only=1")
             return conn
         if self.is_memory:
-            return sqlite3.connect(":memory:")
+            return sqlite3.connect(":memory:", uri=True)
         # mode=ro or immutable=1?
         if self.is_mutable:
             qs = "?mode=ro"
@@ -243,6 +246,12 @@ class Database:
             return None
         return Path(self.path).stat().st_mtime_ns
 
+    async def attached_databases(self):
+        results = await self.execute(
+            "select seq, name, file from pragma_database_list() where seq > 0"
+        )
+        return [AttachedDatabase(*row) for row in results.rows]
+
     async def table_exists(self, table):
         results = await self.execute(
             "select 1 from sqlite_master where type='table' and name=?", params=(table,)