[Patchew-devel] [PATCH 2/2] work around bug in postgresql full text search

[Paolo Bonzini]

The GIN index is not used when searching for keywords.  Hack around
this bug in Postgres.

Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
---
 api/search.py | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/api/search.py b/api/search.py
index 488afa1..66f78b9 100644
--- a/api/search.py
+++ b/api/search.py
@@ -14,7 +14,7 @@ from functools import reduce
 from django.db import connection
 from django.db.models import Q
 
-from django.contrib.postgres.search import SearchQuery, SearchVector
+from django.contrib.postgres.search import SearchQuery, SearchVector, SearchVectorField
 from django.db.models import Lookup, lookups
 from django.db.models.fields import Field
 
@@ -33,6 +33,20 @@ class NotEqual(Lookup):
 class InvalidSearchTerm(Exception):
     pass
 
+# Hack alert: Django wraps each argument to to_tsvector with a COALESCE function,
+# and that causes postgres not to use the index.  Monkeypatch the constructor
+# to skip that step, which we do not need since the subject field is not nullable.
+class NonNullSearchVector(SearchVector):
+    function = 'to_tsvector'
+    arg_joiner = " || ' ' || "
+    _output_field = SearchVectorField()
+    config = None
+
+    def __init__(self, *expressions, **extra):
+        super(SearchVector, self).__init__(*expressions, **extra)
+        self.config = self.extra.get('config', self.config)
+        self.weight = None
+
 
 class SearchEngine(object):
     """
@@ -346,7 +360,7 @@ Search text keyword in the email message. Example:
             queryset = Message.objects.series_heads()
         if self._last_keywords:
             if connection.vendor == 'postgresql':
-                queryset = queryset.annotate(subjsearch=SearchVector('subject', config='english'))
+                queryset = queryset.annotate(subjsearch=NonNullSearchVector('subject', config='english'))
                 searchq = reduce(lambda x, y: x & SearchQuery(y,config='english'),
                                  self._last_keywords)
                 q = q & Q(subjsearch=searchq)
-- 
2.19.1