WIP extra query column information, refs #1293

datasette/templates/query.html

@@ -67,6 +67,8 @@
     </p>
 </form>
 
+extra_column_info: {{ extra_column_info }}
+
 {% if display_rows %}
 <p class="export-links">This data as {% for name, url in renderers.items() %}<a href="{{ url }}">{{ name }}</a>{{ ", " if not loop.last }}{% endfor %}, <a href="{{ url_csv }}">CSV</a></p>
 <div class="table-wrapper"><table class="rows-and-columns">

datasette/utils/__init__.py

@@ -1089,3 +1089,44 @@ async def derive_named_parameters(db, sql):
         return [row["p4"].lstrip(":") for row in results if row["opcode"] == "Variable"]
     except sqlite3.DatabaseError:
         return possible_params
+
+
+def columns_for_query(conn, sql, params=None):
+    """
+    Given a SQLite connection ``conn`` and a SQL query ``sql``,
+    returns a list of ``(table_name, column_name)`` pairs, one
+    per returned column. ``(None, None)`` if no table and column
+    could be derived.
+    """
+    rows = conn.execute("explain " + sql, params).fetchall()
+    table_rootpage_by_register = {
+        r["p1"]: r["p2"] for r in rows if r["opcode"] == "OpenRead"
+    }
+    names_by_rootpage = dict(
+        conn.execute(
+            "select rootpage, name from sqlite_master where rootpage in ({})".format(
+                ", ".join(map(str, table_rootpage_by_register.values()))
+            )
+        )
+    )
+    columns_by_column_register = {}
+    for row in rows:
+        if row["opcode"] in ("Rowid", "Column"):
+            addr, opcode, table_id, cid, column_register, p4, p5, comment = row
+            table = names_by_rootpage[table_rootpage_by_register[table_id]]
+            columns_by_column_register[column_register] = (table, cid)
+    result_row = [dict(r) for r in rows if r["opcode"] == "ResultRow"][0]
+    registers = list(range(result_row["p1"], result_row["p1"] + result_row["p2"]))
+    all_column_names = {}
+    for table in names_by_rootpage.values():
+        table_xinfo = conn.execute("pragma table_xinfo({})".format(table)).fetchall()
+        for row in table_xinfo:
+            all_column_names[(table, row["cid"])] = row["name"]
+    final_output = []
+    for r in registers:
+        try:
+            table, cid = columns_by_column_register[r]
+            final_output.append((table, all_column_names[table, cid]))
+        except KeyError:
+            final_output.append((None, None))
+    return final_output

datasette/views/database.py

@@ -10,6 +10,7 @@ import markupsafe
 from datasette.utils import (
     await_me_maybe,
     check_visibility,
+    columns_for_query,
     derive_named_parameters,
     to_css_class,
     validate_sql_select,
@@ -248,6 +249,8 @@ class QueryView(DataView):
 
         query_error = None
 
+        extra_column_info = None
+
         # Execute query - as write or as read
         if write:
             if request.method == "POST":
@@ -334,6 +337,12 @@ class QueryView(DataView):
                     database, sql, params_for_query, truncate=True, **extra_args
                 )
                 columns = [r[0] for r in results.description]
+
+                # Try to figure out extra column information
+                db = self.ds.get_database(database)
+                extra_column_info = await db.execute_fn(
+                    lambda conn: columns_for_query(conn, sql, params_for_query)
+                )
             except sqlite3.DatabaseError as e:
                 query_error = e
                 results = None
@@ -462,6 +471,7 @@ class QueryView(DataView):
                 "show_hide_text": show_hide_text,
                 "show_hide_hidden": markupsafe.Markup(show_hide_hidden),
                 "hide_sql": hide_sql,
+                "extra_column_info": extra_column_info,
             }
 
         return (