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, 37 insertions, 0 deletions

diff --git a/src/include/access/multixact.h b/src/include/access/multixact.h
new file mode 100644
index 0000000000..1eafddbe83
--- /dev/null
+++ b/src/include/access/multixact.h
@@ -0,0 +1,37 @@
+/*
+ * multixact.h
+ *
+ * PostgreSQL multi-transaction-log manager
+ *
+ * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * $PostgreSQL: pgsql/src/include/access/multixact.h,v 1.1 2005/04/28 21:47:17 tgl Exp $
+ */
+#ifndef MULTIXACT_H
+#define MULTIXACT_H
+
+#define InvalidMultiXactId	((MultiXactId) 0)
+#define FirstMultiXactId	((MultiXactId) 1)
+
+#define MultiXactIdIsValid(multi) ((multi) != InvalidMultiXactId)
+
+extern void MultiXactIdWait(MultiXactId multi);
+extern MultiXactId MultiXactIdExpand(MultiXactId multi, bool isMulti,
+									 TransactionId xid);
+extern bool MultiXactIdIsRunning(MultiXactId multi);
+extern void MultiXactIdSetOldestMember(void);
+
+extern void AtEOXact_MultiXact(void);
+
+extern int	MultiXactShmemSize(void);
+extern void MultiXactShmemInit(void);
+extern void BootStrapMultiXact(void);
+extern void StartupMultiXact(void);
+extern void ShutdownMultiXact(void);
+extern MultiXactId MultiXactGetCheckptMulti(bool is_shutdown);
+extern void CheckPointMultiXact(void);
+extern void MultiXactSetNextMXact(MultiXactId nextMulti);
+extern void MultiXactAdvanceNextMXact(MultiXactId minMulti);
+
+#endif   /* MULTIXACT_H */