Fix very slow user data table query

by adding an index on extension.
Fixes extremely slow tool form building.

lib/galaxy/model/__init__.py

@@ -10910,7 +10910,7 @@ def __repr__(self):
     Column("blurb", TrimmedString(255)),
     Column("peek", TEXT, key="_peek"),
     Column("tool_version", TEXT),
-    Column("extension", TrimmedString(64)),
+    Column("extension", TrimmedString(64), index=True),
     Column("metadata", MetadataType, key="_metadata"),
     Column("metadata_deferred", Boolean, key="metadata_deferred"),
     Column("parent_id", Integer, ForeignKey("history_dataset_association.id"), nullable=True),

lib/galaxy/model/migrations/alembic/versions_gxy/55f02fd8ab6c_add_index_on_hda_extension.py

@@ -0,0 +1,33 @@
+"""Add index on history_dataset_association extension
+
+Revision ID: 55f02fd8ab6c
+Revises: 2dc3386d091f
+Create Date: 2024-03-25 11:14:40.005394
+
+"""
+
+from galaxy.model.database_object_names import build_index_name
+from galaxy.model.migrations.util import (
+    create_index,
+    drop_index,
+)
+
+
+# revision identifiers, used by Alembic.
+revision = "55f02fd8ab6c"
+down_revision = "2dc3386d091f"
+branch_labels = None
+depends_on = None
+
+
+hda_table_name = "history_dataset_association"
+hda_extension_column_name = "extension"
+hda_extension_index_name = build_index_name(hda_table_name, hda_extension_column_name)
+
+
+def upgrade():
+    create_index(hda_extension_index_name, hda_table_name, [hda_extension_column_name])
+
+
+def downgrade():
+    drop_index(hda_extension_index_name, hda_table_name)