Declare the dependency between element type and corpus IDs in PostgreSQL stats

arkindex/documents/migrations/0066_element_type_implies_corpus.py

+# Generated by Django 4.1.7 on 2023-05-23 12:42
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('documents', '0065_elementpath_constraints'),
+    ]
+
+    operations = [
+        # This gives a hint to PostgreSQL's query planner that the element's corpus can be deduced from the type.
+        # If you filter by a specific type ID, then the corpus ID will be constant;
+        # all elements of the same type ID should be in the same corpus.
+        #
+        # This causes the query planner to create significantly better query plans when filtering by both corpus and type ID,
+        # as it understands that filtering on both type and corpus is the same as only filtering on the type,
+        # and the corpus filter will not reduce the planner's estimated row counts.
+        #
+        # On freshly created databases, this applies immediately. On existing databases, this will only have an effect after
+        # an autovacuum completes or after an administrator runs `ANALYZE documents_element`.
+        #
+        # See https://gitlab.com/teklia/arkindex/backend/-/issues/1309#note_1401556847 for context.
+
+        migrations.RunSQL(
+            "CREATE STATISTICS element_type_implies_corpus (dependencies) ON type_id, corpus_id FROM documents_element",
+            "DROP STATISTICS element_type_implies_corpus",
+            elidable=False,
+        ),
+    ]