Implement sharable row-level locks, and use them for foreign key references

to eliminate unnecessary deadlocks.  This commit adds SELECT ... FOR SHARE
paralleling SELECT ... FOR UPDATE.  The implementation uses a new SLRU
data structure (managed much like pg_subtrans) to represent multiple-
transaction-ID sets.  When more than one transaction is holding a shared
lock on a particular row, we create a MultiXactId representing that set
of transactions and store its ID in the row's XMAX.  This scheme allows
an effectively unlimited number of row locks, just as we did before,
while not costing any extra overhead except when a shared lock actually
has to be shared.   Still TODO: use the regular lock manager to control
the grant order when multiple backends are waiting for a row lock.

Alvaro Herrera and Tom Lane.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/include/c.h b/src/include/c.h
index ebed0ab945..31d843fde7 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -12,7 +12,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/c.h,v 1.181 2005/03/29 00:17:16 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/c.h,v 1.182 2005/04/28 21:47:17 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -365,7 +365,8 @@ typedef float float4;
 typedef double float8;
 
 /*
- * Oid, RegProcedure, TransactionId, SubTransactionId, CommandId, AclId
+ * Oid, RegProcedure, TransactionId, SubTransactionId, MultiXactId,
+ * CommandId, AclId
  */
 
 /* typedef Oid is in postgres_ext.h */
@@ -384,6 +385,9 @@ typedef uint32 SubTransactionId;
 #define InvalidSubTransactionId		((SubTransactionId) 0)
 #define TopSubTransactionId			((SubTransactionId) 1)
 
+/* MultiXactId must be equivalent to TransactionId, to fit in t_xmax */
+typedef TransactionId MultiXactId;
+
 typedef uint32 CommandId;
 
 #define FirstCommandId	((CommandId) 0)