Source: rows.go in package github.com/lib/pq

package pq

import (
	"math"
	"reflect"
	"time"

	"github.com/lib/pq/oid"
)

const headerSize = 4

The object ID of the data type.

The data type size (see pg_type.typlen). Note that negative values denote variable-width types.

The type modifier (see pg_attribute.atttypmod). The meaning of the modifier is type-specific.

	Mod int
}

func (fd fieldDesc) Type() reflect.Type {
	switch fd.OID {
	case oid.T_int8:
		return reflect.TypeOf(int64(0))
	case oid.T_int4:
		return reflect.TypeOf(int32(0))
	case oid.T_int2:
		return reflect.TypeOf(int16(0))
	case oid.T_varchar, oid.T_text:
		return reflect.TypeOf("")
	case oid.T_bool:
		return reflect.TypeOf(false)
	case oid.T_date, oid.T_time, oid.T_timetz, oid.T_timestamp, oid.T_timestamptz:
		return reflect.TypeOf(time.Time{})
	case oid.T_bytea:
		return reflect.TypeOf([]byte(nil))
	default:
		return reflect.TypeOf(new(interface{})).Elem()
	}
}

func (fd fieldDesc) Name() string {
	return oid.TypeName[fd.OID]
}

func (fd fieldDesc) Length() (length int64, ok bool) {
	switch fd.OID {
	case oid.T_text, oid.T_bytea:
		return math.MaxInt64, true
	case oid.T_varchar, oid.T_bpchar:
		return int64(fd.Mod - headerSize), true
	default:
		return 0, false
	}
}

func (fd fieldDesc) PrecisionScale() (precision, scale int64, ok bool) {
	switch fd.OID {
	case oid.T_numeric, oid.T__numeric:
		mod := fd.Mod - headerSize
		precision = int64((mod >> 16) & 0xffff)
		scale = int64(mod & 0xffff)
		return precision, scale, true
	default:
		return 0, 0, false
	}
}

ColumnTypeScanType returns the value type that can be used to scan types into.

func (rs *rows) ColumnTypeScanType(index int) reflect.Type {
	return rs.colTyps[index].Type()
}

ColumnTypeDatabaseTypeName return the database system type name.

func (rs *rows) ColumnTypeDatabaseTypeName(index int) string {
	return rs.colTyps[index].Name()
}

ColumnTypeLength returns the length of the column type if the column is a variable length type. If the column is not a variable length type ok should return false.

func (rs *rows) ColumnTypeLength(index int) (length int64, ok bool) {
	return rs.colTyps[index].Length()
}

ColumnTypePrecisionScale should return the precision and scale for decimal types. If not applicable, ok should be false.

func (rs *rows) ColumnTypePrecisionScale(index int) (precision, scale int64, ok bool) {
	return rs.colTyps[index].PrecisionScale()