Add insert --truncate option

Deletes all rows in the table (if it exists) before inserting new rows.
SQLite doesn't implement a TRUNCATE TABLE statement but does optimize an
unqualified DELETE FROM.

This can be handy if you want to refresh the entire contents of a table
but a) don't have a PK (so can't use --replace) and b) don't want the
table to disappear (even briefly) for other connections.  The
replacement of rows should appear instantaneous to other connections
since the DELETE + INSERT are executed in a transaction.

sqlite_utils/cli.py

@@ -423,6 +423,7 @@ def insert_upsert_implementation(
     upsert,
     ignore=False,
     replace=False,
+    truncate=False,
     not_null=None,
     default=None,
 ):
@@ -442,7 +443,7 @@ def insert_upsert_implementation(
         docs = json.load(json_file)
         if isinstance(docs, dict):
             docs = [docs]
-    extra_kwargs = {"ignore": ignore, "replace": replace}
+    extra_kwargs = {"ignore": ignore, "replace": replace, "truncate": truncate}
     if not_null:
         extra_kwargs["not_null"] = set(not_null)
     if default:
@@ -465,6 +466,12 @@ def insert_upsert_implementation(
     default=False,
     help="Replace records if pk already exists",
 )
+@click.option(
+    "--truncate",
+    is_flag=True,
+    default=False,
+    help="Truncate table before inserting records, if table already exists",
+)
 def insert(
     path,
     table,
@@ -477,6 +484,7 @@ def insert(
     alter,
     ignore,
     replace,
+    truncate,
     not_null,
     default,
 ):
@@ -499,6 +507,7 @@ def insert(
         upsert=False,
         ignore=ignore,
         replace=replace,
+        truncate=truncate,
         not_null=not_null,
         default=default,
     )

sqlite_utils/db.py

@@ -976,6 +976,7 @@ def insert_all(
         alter=DEFAULT,
         ignore=DEFAULT,
         replace=DEFAULT,
+        truncate=False,
         extracts=DEFAULT,
         conversions=DEFAULT,
         columns=DEFAULT,
@@ -1032,6 +1033,8 @@ def insert_all(
             # issue implicit BEGINs for DDL, only DML.  We mix DDL and DML
             # below and might execute DDL first, e.g. for table creation.
             self.db.conn.execute("BEGIN")
+            if truncate and self.exists():
+                self.db.conn.execute("DELETE FROM [{}];".format(self.name))
             for chunk in chunks(itertools.chain([first_record], records), batch_size):
                 chunk = list(chunk)
                 num_records_processed += len(chunk)

tests/test_cli.py

@@ -642,6 +642,31 @@ def test_insert_replace(db_path, tmpdir):
     )
 
 
+def test_insert_truncate(db_path):
+    result = CliRunner().invoke(
+        cli.cli,
+        ["insert", db_path, "from_json_nl", "-", "--nl", "--batch-size=1"],
+        input='{"foo": "bar", "n": 1}\n{"foo": "baz", "n": 2}',
+    )
+    assert 0 == result.exit_code, result.output
+    db = Database(db_path)
+    assert [
+        {"foo": "bar", "n": 1},
+        {"foo": "baz", "n": 2},
+    ] == db.execute_returning_dicts("select foo, n from from_json_nl")
+    # Truncate and insert new rows
+    result = CliRunner().invoke(
+        cli.cli,
+        ["insert", db_path, "from_json_nl", "-", "--nl", "--truncate", "--batch-size=1"],
+        input='{"foo": "bam", "n": 3}\n{"foo": "bat", "n": 4}',
+    )
+    assert 0 == result.exit_code, result.output
+    assert [
+        {"foo": "bam", "n": 3},
+        {"foo": "bat", "n": 4},
+    ] == db.execute_returning_dicts("select foo, n from from_json_nl")
+
+
 def test_insert_alter(db_path, tmpdir):
     result = CliRunner().invoke(
         cli.cli,