BeginCopyFrom(Relation rel,
			  const char *filename,
			  bool is_program,
			  List *attnamelist,
			  List *options)
{
	CopyState	cstate;
	bool		pipe = (filename == NULL);
	TupleDesc	tupDesc;
	Form_pg_attribute *attr;
	AttrNumber	num_phys_attrs,
				num_defaults;
	FmgrInfo   *in_functions;
	Oid		   *typioparams;
	int			attnum;
	Oid			in_func_oid;
	int		   *defmap;
	ExprState **defexprs;
	MemoryContext oldcontext;
	bool		volatile_defexprs;

	cstate = BeginCopy(true, rel, NULL, NULL, InvalidOid, attnamelist, options);
	oldcontext = MemoryContextSwitchTo(cstate->copycontext);

	/* Initialize state variables */
	cstate->fe_eof = false;
	cstate->eol_type = EOL_UNKNOWN;
	cstate->cur_relname = RelationGetRelationName(cstate->rel);
	cstate->cur_lineno = 0;
	cstate->cur_attname = NULL;
	cstate->cur_attval = NULL;

	/* Set up variables to avoid per-attribute overhead. */
	initStringInfo(&cstate->attribute_buf);
	initStringInfo(&cstate->line_buf);
	cstate->line_buf_converted = false;
	cstate->raw_buf = (char *) palloc(RAW_BUF_SIZE + 1);
	cstate->raw_buf_index = cstate->raw_buf_len = 0;

	tupDesc = RelationGetDescr(cstate->rel);
	attr = tupDesc->attrs;
	num_phys_attrs = tupDesc->natts;
	num_defaults = 0;
	volatile_defexprs = false;

	/*
	 * Pick up the required catalog information for each attribute in the
	 * relation, including the input function, the element type (to pass to
	 * the input function), and info about defaults and constraints. (Which
	 * input function we use depends on text/binary format choice.)
	 */
	in_functions = (FmgrInfo *) palloc(num_phys_attrs * sizeof(FmgrInfo));
	typioparams = (Oid *) palloc(num_phys_attrs * sizeof(Oid));
	defmap = (int *) palloc(num_phys_attrs * sizeof(int));
	defexprs = (ExprState **) palloc(num_phys_attrs * sizeof(ExprState *));

	for (attnum = 1; attnum <= num_phys_attrs; attnum++)
	{
		/* We don't need info for dropped attributes */
		if (attr[attnum - 1]->attisdropped)
			continue;

		/* Fetch the input function and typioparam info */
		if (cstate->binary)
			getTypeBinaryInputInfo(attr[attnum - 1]->atttypid,
								   &in_func_oid, &typioparams[attnum - 1]);
		else
			getTypeInputInfo(attr[attnum - 1]->atttypid,
							 &in_func_oid, &typioparams[attnum - 1]);
		fmgr_info(in_func_oid, &in_functions[attnum - 1]);

		/* Get default info if needed */
		if (!list_member_int(cstate->attnumlist, attnum))
		{
			/* attribute is NOT to be copied from input */
			/* use default value if one exists */
			Expr	   *defexpr = (Expr *) build_column_default(cstate->rel,
																attnum);

			if (defexpr != NULL)
			{
				/* Run the expression through planner */
				defexpr = expression_planner(defexpr);

				/* Initialize executable expression in copycontext */
				defexprs[num_defaults] = ExecInitExpr(defexpr, NULL);
				defmap[num_defaults] = attnum - 1;
				num_defaults++;

				/*
				 * If a default expression looks at the table being loaded,
				 * then it could give the wrong answer when using
				 * multi-insert. Since database access can be dynamic this is
				 * hard to test for exactly, so we use the much wider test of
				 * whether the default expression is volatile. We allow for
				 * the special case of when the default expression is the
				 * nextval() of a sequence which in this specific case is
				 * known to be safe for use with the multi-insert
				 * optimisation. Hence we use this special case function
				 * checker rather than the standard check for
				 * contain_volatile_functions().
				 */
				if (!volatile_defexprs)
					volatile_defexprs = contain_volatile_functions_not_nextval((Node *) defexpr);
			}
		}
	}

	/* We keep those variables in cstate. */
	cstate->in_functions = in_functions;
	cstate->typioparams = typioparams;
	cstate->defmap = defmap;
	cstate->defexprs = defexprs;
	cstate->volatile_defexprs = volatile_defexprs;
	cstate->num_defaults = num_defaults;
	cstate->is_program = is_program;

	if (pipe)
	{
		Assert(!is_program);	/* the grammar does not allow this */
		if (whereToSendOutput == DestRemote)
			ReceiveCopyBegin(cstate);
		else
			cstate->copy_file = stdin;
	}
	else
	{
		cstate->filename = pstrdup(filename);

		if (cstate->is_program)
		{
			cstate->copy_file = OpenPipeStream(cstate->filename, PG_BINARY_R);
			if (cstate->copy_file == NULL)
				ereport(ERROR,
						(errmsg("could not execute command \"%s\": %m",
								cstate->filename)));
		}
		else
		{
			struct stat st;

			cstate->copy_file = AllocateFile(cstate->filename, PG_BINARY_R);
			if (cstate->copy_file == NULL)
				ereport(ERROR,
						(errcode_for_file_access(),
						 errmsg("could not open file \"%s\" for reading: %m",
								cstate->filename)));

			if (fstat(fileno(cstate->copy_file), &st))
				elog(ERROR, "could not stat file \"%s\": %m", cstate->filename);

			if (S_ISDIR(st.st_mode))
				ereport(ERROR,
						(errcode(ERRCODE_WRONG_OBJECT_TYPE),
						 errmsg("\"%s\" is a directory", cstate->filename)));
		}
	}

	if (!cstate->binary)
	{
		/* must rely on user to tell us... */
		cstate->file_has_oids = cstate->oids;
	}
	else
	{
		/* Read and verify binary header */
		char		readSig[11];
		int32		tmp;

		/* Signature */
		if (CopyGetData(cstate, readSig, 11, 11) != 11 ||
			memcmp(readSig, BinarySignature, 11) != 0)
			ereport(ERROR,
					(errcode(ERRCODE_BAD_COPY_FILE_FORMAT),
					 errmsg("COPY file signature not recognized")));
		/* Flags field */
		if (!CopyGetInt32(cstate, &tmp))
			ereport(ERROR,
					(errcode(ERRCODE_BAD_COPY_FILE_FORMAT),
					 errmsg("invalid COPY file header (missing flags)")));
		cstate->file_has_oids = (tmp & (1 << 16)) != 0;
		tmp &= ~(1 << 16);
		if ((tmp >> 16) != 0)
			ereport(ERROR,
					(errcode(ERRCODE_BAD_COPY_FILE_FORMAT),
				 errmsg("unrecognized critical flags in COPY file header")));
		/* Header extension length */
		if (!CopyGetInt32(cstate, &tmp) ||
			tmp < 0)
			ereport(ERROR,
					(errcode(ERRCODE_BAD_COPY_FILE_FORMAT),
					 errmsg("invalid COPY file header (missing length)")));
		/* Skip extension header, if present */
		while (tmp-- > 0)
		{
			if (CopyGetData(cstate, readSig, 1, 1) != 1)
				ereport(ERROR,
						(errcode(ERRCODE_BAD_COPY_FILE_FORMAT),
						 errmsg("invalid COPY file header (wrong length)")));
		}
	}

	if (cstate->file_has_oids && cstate->binary)
	{
		getTypeBinaryInputInfo(OIDOID,
							   &in_func_oid, &cstate->oid_typioparam);
		fmgr_info(in_func_oid, &cstate->oid_in_function);
	}

	/* create workspace for CopyReadAttributes results */
	if (!cstate->binary)
	{
		AttrNumber	attr_count = list_length(cstate->attnumlist);
		int			nfields = cstate->file_has_oids ? (attr_count + 1) : attr_count;

		cstate->max_fields = nfields;
		cstate->raw_fields = (char **) palloc(nfields * sizeof(char *));
	}

	MemoryContextSwitchTo(oldcontext);

	return cstate;
}