diff --git a/sql/upgrades/128.sql b/sql/upgrades/128.sql
new file mode 100644
--- /dev/null
+++ b/sql/upgrades/128.sql
@@ -0,0 +1,9 @@
+-- SWH Indexer DB schema upgrade
+-- from_version: 127
+-- to_version: 128
+-- description: Add index on content_mimetype table to improve read queries
+
+insert into dbversion(version, release, description)
+values(128, now(), 'Work In Progress');
+
+create index on content_mimetype(id) where mimetype like 'text/%';
diff --git a/swh/indexer/sql/30-swh-schema.sql b/swh/indexer/sql/30-swh-schema.sql
--- a/swh/indexer/sql/30-swh-schema.sql
+++ b/swh/indexer/sql/30-swh-schema.sql
@@ -14,7 +14,7 @@
 );
 
 insert into dbversion(version, release, description)
-      values(127, now(), 'Work In Progress');
+      values(128, now(), 'Work In Progress');
 -- Computing metadata on sha1's contents
 
 -- a SHA1 checksum (not necessarily originating from Git)
diff --git a/swh/indexer/sql/60-swh-indexes.sql b/swh/indexer/sql/60-swh-indexes.sql
--- a/swh/indexer/sql/60-swh-indexes.sql
+++ b/swh/indexer/sql/60-swh-indexes.sql
@@ -39,6 +39,8 @@
 alter table content_mimetype add constraint content_mimetype_indexer_configuration_id_fkey foreign key (indexer_configuration_id) references indexer_configuration(id) not valid;
 alter table content_mimetype validate constraint content_mimetype_indexer_configuration_id_fkey;
 
+create index on content_mimetype(id) where mimetype like 'text/%';
+
 -- content_language
 create unique index content_language_pkey on content_language(id, indexer_configuration_id);
 alter table content_language add primary key using index content_language_pkey;
diff --git a/swh/indexer/storage/db.py b/swh/indexer/storage/db.py
--- a/swh/indexer/storage/db.py
+++ b/swh/indexer/storage/db.py
@@ -134,19 +134,24 @@
         table = self.content_indexer_names[content_type]
         if with_textual_data:
             extra = """inner join content_mimetype cm
-                         on (t.id=cm.id and cm.mimetype like 'text/%%')"""
+                         on (t.id=cm.id and cm.mimetype like 'text/%%' and
+                             %(start)s <= cm.id and cm.id <= %(end)s)
+                    """
         else:
             extra = ""
-        query = """select t.id
-                   from %s t
-                   inner join indexer_configuration ic
-                   on t.indexer_configuration_id=ic.id
-                   %s
-                   where ic.id=%%s and
-                         %%s <= t.id and t.id <= %%s
-                   order by t.indexer_configuration_id, t.id
-                   limit %%s""" % (table, extra)
-        cur.execute(query, (indexer_configuration_id, start, end, limit))
+        query = f"""select t.id
+                    from {table} t
+                    {extra}
+                    where t.indexer_configuration_id=%(tool_id)s
+                          and %(start)s <= t.id and t.id <= %(end)s
+                    order by t.indexer_configuration_id, t.id
+                    limit %(limit)s"""
+        cur.execute(query, {
+            'start': start,
+            'end': end,
+            'tool_id': indexer_configuration_id,
+            'limit': limit,
+        })
         yield from cur
 
     def content_mimetype_get_from_list(self, ids, cur=None):