Add a DEFAULT option to COPY FROM

This allows for a string which if an input field matches causes the
column's default value to be inserted. The advantage of this is that
the default can be inserted in some rows and not others, for which
non-default data is available.

The file_fdw extension is also modified to take allow use of this
option.

Israel Barth Rubio

Discussion: https://postgr.es/m/CAO_rXXAcqesk6DsvioOZ5zmeEmpUN5ktZf-9=9yu+DTr0Xr8Uw@mail.gmail.com

src/include/commands/copyfrom_internal.h

@@ -44,8 +44,7 @@ typedef enum EolType
  */
 typedef enum CopyInsertMethod
 {
-	CIM_SINGLE,					/* use table_tuple_insert or
-								 * ExecForeignInsert */
+	CIM_SINGLE,					/* use table_tuple_insert or ExecForeignInsert */
 	CIM_MULTI,					/* always use table_multi_insert or
 								 * ExecForeignBatchInsert */
 	CIM_MULTI_CONDITIONAL		/* use table_multi_insert or
@@ -91,11 +90,16 @@ typedef struct CopyFromStateData
 	 */
 	MemoryContext copycontext;	/* per-copy execution context */
 
-	AttrNumber	num_defaults;
+	AttrNumber	num_defaults;	/* count of att that are missing and have
+								 * default value */
 	FmgrInfo   *in_functions;	/* array of input functions for each attrs */
 	Oid		   *typioparams;	/* array of element types for in_functions */
-	int		   *defmap;			/* array of default att numbers */
-	ExprState **defexprs;		/* array of default att expressions */
+	int		   *defmap;			/* array of default att numbers related to
+								 * missing att */
+	ExprState **defexprs;		/* array of default att expressions for all
+								 * att */
+	bool	   *defaults;		/* if DEFAULT marker was found for
+								 * corresponding att */
 	bool		volatile_defexprs;	/* is any of defexprs volatile? */
 	List	   *range_table;	/* single element list of RangeTblEntry */
 	List	   *rteperminfos;	/* single element list of RTEPermissionInfo */