Add support for asynchronous execution.

This implements asynchronous execution, which runs multiple parts of a
non-parallel-aware Append concurrently rather than serially to improve
performance when possible.  Currently, the only node type that can be
run concurrently is a ForeignScan that is an immediate child of such an
Append.  In the case where such ForeignScans access data on different
remote servers, this would run those ForeignScans concurrently, and
overlap the remote operations to be performed simultaneously, so it'll
improve the performance especially when the operations involve
time-consuming ones such as remote join and remote aggregation.

We may extend this to other node types such as joins or aggregates over
ForeignScans in the future.

This also adds the support for postgres_fdw, which is enabled by the
table-level/server-level option "async_capable".  The default is false.

Robert Haas, Kyotaro Horiguchi, Thomas Munro, and myself.  This commit
is mostly based on the patch proposed by Robert Haas, but also uses
stuff from the patch proposed by Kyotaro Horiguchi and from the patch
proposed by Thomas Munro.  Reviewed by Kyotaro Horiguchi, Konstantin
Knizhnik, Andrey Lepikhov, Movead Li, Thomas Munro, Justin Pryzby, and
others.

Discussion: https://postgr.es/m/CA%2BTgmoaXQEt4tZ03FtQhnzeDEMzBck%2BLrni0UWHVVgOTnA6C1w%40mail.gmail.com
Discussion: https://postgr.es/m/CA%2BhUKGLBRyu0rHrDCMC4%3DRn3252gogyp1SjOgG8SEKKZv%3DFwfQ%40mail.gmail.com
Discussion: https://postgr.es/m/20200228.170650.667613673625155850.horikyota.ntt%40gmail.com

1 files changed, 15 insertions, 2 deletions

diff --git a/contrib/postgres_fdw/postgres_fdw.h b/contrib/postgres_fdw/postgres_fdw.h
index 1f67b4d9fd2..88d94da6f6b 100644
--- a/contrib/postgres_fdw/postgres_fdw.h
+++ b/contrib/postgres_fdw/postgres_fdw.h
@@ -16,6 +16,7 @@
 #include "foreign/foreign.h"
 #include "lib/stringinfo.h"
 #include "libpq-fe.h"
+#include "nodes/execnodes.h"
 #include "nodes/pathnodes.h"
 #include "utils/relcache.h"
 
@@ -78,6 +79,7 @@ typedef struct PgFdwRelationInfo
 	Cost		fdw_startup_cost;
 	Cost		fdw_tuple_cost;
 	List	   *shippable_extensions;	/* OIDs of shippable extensions */
+	bool		async_capable;
 
 	/* Cached catalog information. */
 	ForeignTable *table;
@@ -124,17 +126,28 @@ typedef struct PgFdwRelationInfo
 	int			relation_index;
 } PgFdwRelationInfo;
 
+/*
+ * Extra control information relating to a connection.
+ */
+typedef struct PgFdwConnState
+{
+	AsyncRequest *pendingAreq;	/* pending async request */
+} PgFdwConnState;
+
 /* in postgres_fdw.c */
 extern int	set_transmission_modes(void);
 extern void reset_transmission_modes(int nestlevel);
+extern void process_pending_request(AsyncRequest *areq);
 
 /* in connection.c */
-extern PGconn *GetConnection(UserMapping *user, bool will_prep_stmt);
+extern PGconn *GetConnection(UserMapping *user, bool will_prep_stmt,
+							 PgFdwConnState **state);
 extern void ReleaseConnection(PGconn *conn);
 extern unsigned int GetCursorNumber(PGconn *conn);
 extern unsigned int GetPrepStmtNumber(PGconn *conn);
 extern PGresult *pgfdw_get_result(PGconn *conn, const char *query);
-extern PGresult *pgfdw_exec_query(PGconn *conn, const char *query);
+extern PGresult *pgfdw_exec_query(PGconn *conn, const char *query,
+								  PgFdwConnState *state);
 extern void pgfdw_report_error(int elevel, PGresult *res, PGconn *conn,
 							   bool clear, const char *sql);