Improve error messages for query argument encoding

conn.go

@@ -488,6 +488,7 @@ func (c *Conn) execParamsAndPreparedPrefix(sd *pgconn.StatementDescription, args
 	for i := range args {
 		err := c.eqb.AppendParam(c.typeMap, sd.ParamOIDs[i], args[i])
 		if err != nil {
+			err = fmt.Errorf("failed to encode args[%d]: %v", i, err)
 			return err
 		}
 	}
@@ -739,6 +740,7 @@ optionLoop:
 		for i := range args {
 			err = c.eqb.AppendParam(c.typeMap, sd.ParamOIDs[i], args[i])
 			if err != nil {
+				err = fmt.Errorf("failed to encode args[%d]: %v", i, err)
 				rows.fatal(err)
 				return rows, rows.err
 			}
@@ -895,6 +897,7 @@ func (c *Conn) SendBatch(ctx context.Context, b *Batch) BatchResults {
 				for i := range bi.arguments {
 					err := c.eqb.AppendParam(c.typeMap, sd.ParamOIDs[i], bi.arguments[i])
 					if err != nil {
+						err = fmt.Errorf("failed to encode args[%d]: %v", i, err)
 						return &batchResults{ctx: ctx, conn: c, err: err}
 					}
 				}
@@ -962,6 +965,7 @@ func (c *Conn) SendBatch(ctx context.Context, b *Batch) BatchResults {
 			for i := range bi.arguments {
 				err := c.eqb.AppendParam(c.typeMap, sd.ParamOIDs[i], bi.arguments[i])
 				if err != nil {
+					err = fmt.Errorf("failed to encode args[%d]: %v", i, err)
 					return &batchResults{ctx: ctx, conn: c, err: err}
 				}
 			}

pgtype/pgtype.go

@@ -1737,7 +1737,14 @@ func (m *Map) Encode(oid uint32, formatCode int16, value interface{}, buf []byte
 			return m.Encode(oid, formatCode, v, buf)
 		}
 
-		return nil, fmt.Errorf("unable to encode %#v into OID %d", value, oid)
+		return nil, fmt.Errorf("unable to encode %#v into format code %d for OID %d", value, formatCode, oid)
 	}
-	return plan.Encode(value, buf)
+
+	newBuf, err = plan.Encode(value, buf)
+	if err != nil {
+		err = fmt.Errorf("unable to encode %#v into format code %d for OID %d: %v", value, formatCode, oid, err)
+		return nil, err
+	}
+
+	return newBuf, nil
 }

query_test.go

@@ -983,7 +983,7 @@ func TestQueryRowErrors(t *testing.T) {
 		{"select $1::badtype", []interface{}{"Jack"}, []interface{}{&actual.i16}, `type "badtype" does not exist`},
 		{"SYNTAX ERROR", []interface{}{}, []interface{}{&actual.i16}, "SQLSTATE 42601"},
 		{"select $1::text", []interface{}{"Jack"}, []interface{}{&actual.i16}, "cannot scan OID 25 in text format into *int16"},
-		{"select $1::point", []interface{}{int(705)}, []interface{}{&actual.s}, "unable to encode 705 into OID 600"},
+		{"select $1::point", []interface{}{int(705)}, []interface{}{&actual.s}, "unable to encode 705 into format code 1 for OID 600"},
 	}
 
 	for i, tt := range tests {