Refactor COPY TO to use format callback functions.

This commit introduces a new CopyToRoutine struct, which is a set of
callback routines to copy tuples in a specific format. It also makes
the existing formats (text, CSV, and binary) utilize these format
callbacks.

This change is a preliminary step towards making the COPY TO command
extensible in terms of output formats.

Additionally, this refactoring contributes to a performance
improvement by reducing the number of "if" branches that need to be
checked on a per-row basis when sending field representations in text
or CSV mode. The performance benchmark results showed ~5% performance
gain in text or CSV mode.

Author: Sutou Kouhei <kou@clear-code.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Discussion: https://postgr.es/m/20231204.153548.2126325458835528809.kou@clear-code.com

1 files changed, 57 insertions, 0 deletions

diff --git a/src/include/commands/copyapi.h b/src/include/commands/copyapi.h
new file mode 100644
index 00000000000..bd2d386816e
--- /dev/null
+++ b/src/include/commands/copyapi.h
@@ -0,0 +1,57 @@
+/*-------------------------------------------------------------------------
+ *
+ * copyapi.h
+ *	  API for COPY TO handlers
+ *
+ *
+ * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/commands/copyapi.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef COPYAPI_H
+#define COPYAPI_H
+
+#include "commands/copy.h"
+
+/*
+ * API structure for a COPY TO format implementation. Note this must be
+ * allocated in a server-lifetime manner, typically as a static const struct.
+ */
+typedef struct CopyToRoutine
+{
+	/*
+	 * Set output function information. This callback is called once at the
+	 * beginning of COPY TO.
+	 *
+	 * 'finfo' can be optionally filled to provide the catalog information of
+	 * the output function.
+	 *
+	 * 'atttypid' is the OID of data type used by the relation's attribute.
+	 */
+	void		(*CopyToOutFunc) (CopyToState cstate, Oid atttypid,
+								  FmgrInfo *finfo);
+
+	/*
+	 * Start a COPY TO. This callback is called once at the beginning of COPY
+	 * TO.
+	 *
+	 * 'tupDesc' is the tuple descriptor of the relation from where the data
+	 * is read.
+	 */
+	void		(*CopyToStart) (CopyToState cstate, TupleDesc tupDesc);
+
+	/*
+	 * Write one row stored in 'slot' to the destination.
+	 */
+	void		(*CopyToOneRow) (CopyToState cstate, TupleTableSlot *slot);
+
+	/*
+	 * End a COPY TO. This callback is called once at the end of COPY TO.
+	 */
+	void		(*CopyToEnd) (CopyToState cstate);
+} CopyToRoutine;
+
+#endif							/* COPYAPI_H */