fix: add an index to FMR.date_modified

`/oai-pmh/?verb=Identify` reports the "earliest datestamp" based on the
FormattedMetadataRecord with the earliest `date_modified` -- but that
query can take > 30 seconds, because there is no index.

add an index.

share/migrations/0059_fmr_date_modified_index.py

@@ -0,0 +1,34 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    atomic = False  # CREATE INDEX CONCURRENTLY cannot be run in a txn
+
+    dependencies = [
+        ('share', '0001_squashed_0058_big_rend'),
+    ]
+
+    operations = [
+        migrations.SeparateDatabaseAndState(
+            state_operations=[
+                # tell Django we're adding an index
+                migrations.AddIndex(
+                    model_name='formattedmetadatarecord',
+                    index=models.Index(fields=['date_modified'], name=['fmr_date_modified_index']),
+                ),
+            ],
+            database_operations=[
+                # add the index without locking
+                # (if we were using Django 3, would use
+                #  django.contrib.postgres.operations.AddIndexConcurrently)
+                migrations.RunSQL([
+                    'CREATE INDEX CONCURRENTLY "fmr_date_modified_index" ON "share_formattedmetadatarecord" ("date_modified");',
+                ], [
+                    'DROP INDEX IF EXISTS "fmr_date_modified_index";'
+                ])
+            ]
+        )
+    ]

share/models/core.py

@@ -252,6 +252,9 @@ class JSONAPIMeta(BaseJSONAPIMeta):
 
     class Meta:
         unique_together = ('suid', 'record_format')
+        indexes = [
+            models.Index(fields=['date_modified'], name=['fmr_date_modified_index'])
+        ]
 
     def __repr__(self):
         return f'<{self.__class__.__name__}({self.id}, {self.suid_id}, {self.record_format})>'