From 027726da70f31541bd3c4813bf464489a808ae91 Mon Sep 17 00:00:00 2001
From: Dan Gudmundsson <dgud@erlang.org>
Date: Thu, 10 Sep 2015 10:09:28 +0200
Subject: mnesia: Improve index updates

There is no need to update the index table if a record is updated in non
indexed field. This removes one timing glitch where dirty_index_read would
return an empty list for records that where updated.

There is still an issue with dirty_index_read when updates are made to
the index field, it have been reduced but the real table updates are made
after the index table references have been added.

Originally reported by Nick Marino in erl-questions mailing list, thanks.
---
 lib/mnesia/src/mnesia_index.erl              | 27 +++++++++++++++++----------
 lib/mnesia/test/mnesia_dirty_access_test.erl | 19 ++++++++++++++++++-
 2 files changed, 35 insertions(+), 11 deletions(-)

(limited to 'lib')

diff --git a/lib/mnesia/src/mnesia_index.erl b/lib/mnesia/src/mnesia_index.erl
index 81ed5a8f1a..0c882c0df6 100644
--- a/lib/mnesia/src/mnesia_index.erl
+++ b/lib/mnesia/src/mnesia_index.erl
@@ -70,17 +70,24 @@ add_index(Index, Tab, Key, Obj, Old) ->
 add_index2([{Pos, Ixt} |Tail], bag, Tab, K, Obj, OldRecs) ->
     db_put(Ixt, {element(Pos, Obj), K}),
     add_index2(Tail, bag, Tab, K, Obj, OldRecs);
-add_index2([{Pos, Ixt} |Tail], Type, Tab, K, Obj, OldRecs) ->
+add_index2([{Pos, Ixt} |Tail], Type, Tab, K, Obj, OldRecs0) ->
     %% Remove old tuples in index if Tab is updated
-    case OldRecs of 
-	undefined -> 
-	    Old = mnesia_lib:db_get(Tab, K),
-	    del_ixes(Ixt, Old, Pos, K);
-	Old -> 
-	    del_ixes(Ixt, Old, Pos, K)
-    end,
-    db_put(Ixt, {element(Pos, Obj), K}),
-    add_index2(Tail, Type, Tab, K, Obj, OldRecs);
+    OldRecs1 = case OldRecs0 of
+		   undefined -> mnesia_lib:db_get(Tab, K);
+		   _ -> OldRecs0
+	       end,
+    IdxVal = element(Pos, Obj),
+    case [Old || Old <- OldRecs1, element(Pos, Old) =/= IdxVal] of
+	[] when OldRecs1 =:= [] ->  %% Write
+	    db_put(Ixt, {element(Pos, Obj), K}),
+	    add_index2(Tail, Type, Tab, K, Obj, OldRecs0);
+	[] -> %% when OldRecs1 =/= [] Update without modifying index field
+	    add_index2(Tail, Type, Tab, K, Obj, OldRecs0);
+	OldRecs -> %% Update
+	    db_put(Ixt, {element(Pos, Obj), K}),
+	    del_ixes(Ixt, OldRecs, Pos, K),
+	    add_index2(Tail, Type, Tab, K, Obj, OldRecs0)
+    end;
 add_index2([], _, _Tab, _K, _Obj, _) -> ok.
 
 delete_index(Index, Tab, K) ->
diff --git a/lib/mnesia/test/mnesia_dirty_access_test.erl b/lib/mnesia/test/mnesia_dirty_access_test.erl
index 89aff2b3b8..0d57e5a1b1 100644
--- a/lib/mnesia/test/mnesia_dirty_access_test.erl
+++ b/lib/mnesia/test/mnesia_dirty_access_test.erl
@@ -401,9 +401,26 @@ dirty_index_read(Config, Storage) ->
     ?match({'EXIT', _},  mnesia:dirty_index_read(Tab, 2, BadValPos)), 
     ?match({'EXIT', _},  mnesia:dirty_index_read(foo, 2, ValPos)), 
     ?match({'EXIT', _},  mnesia:dirty_index_read([], 2, ValPos)), 
-    
+
+    mnesia:dirty_write({Tab, 5, 1}),
+    ?match(ok, index_read_loop(Tab, 0)),
+
     ?verify_mnesia(Nodes, []).
 
+
+index_read_loop(Tab, N) when N =< 1000 ->
+    spawn_link(fun() ->
+		       mnesia:transaction(fun() -> mnesia:write({Tab, 5, 1}) end)
+	       end),
+    case mnesia:dirty_match_object({Tab, '_', 1}) of
+	[{Tab, 5, 1}] ->
+	    index_read_loop(Tab, N+1);
+	Other -> {N, Other}
+    end;
+index_read_loop(_, _) ->
+    ok.
+
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
 
-- 
cgit v1.2.3