Source: values.go in package github.com/jackc/pgx/v4

package pgx

import (
	"database/sql/driver"
	"fmt"
	"math"
	"reflect"
	"time"

	"github.com/jackc/pgio"
	"github.com/jackc/pgtype"
)

PostgreSQL format codes

const (
	TextFormatCode   = 0
	BinaryFormatCode = 1
)

SerializationError occurs on failure to encode or decode a value

type SerializationError string

func (e SerializationError) Error() string {
	return string(e)
}

func convertSimpleArgument(ci *pgtype.ConnInfo, arg interface{}) (interface{}, error) {
	if arg == nil {
		return nil, nil
	}

	refVal := reflect.ValueOf(arg)
	if refVal.Kind() == reflect.Ptr && refVal.IsNil() {
		return nil, nil
	}

	switch arg := arg.(type) {

https://github.com/jackc/pgx/issues/409 Changed JSON and JSONB to surface []byte to database/sql instead of string. But that caused problems with the simple protocol because the driver.Valuer case got taken before the pgtype.TextEncoder case. And driver.Valuer needed to be first in the usual case because of https://github.com/jackc/pgx/issues/339. So instead we special case JSON and JSONB.

	case *pgtype.JSON:
		buf, err := arg.EncodeText(ci, nil)
		if err != nil {
			return nil, err
		}
		if buf == nil {
			return nil, nil
		}
		return string(buf), nil
	case *pgtype.JSONB:
		buf, err := arg.EncodeText(ci, nil)
		if err != nil {
			return nil, err
		}
		if buf == nil {
			return nil, nil
		}
		return string(buf), nil

	case driver.Valuer:
		return callValuerValue(arg)
	case pgtype.TextEncoder:
		buf, err := arg.EncodeText(ci, nil)
		if err != nil {
			return nil, err
		}
		if buf == nil {
			return nil, nil
		}
		return string(buf), nil
	case float32:
		return float64(arg), nil
	case float64:
		return arg, nil
	case bool:
		return arg, nil
	case time.Time:
		return arg, nil
	case string:
		return arg, nil
	case []byte:
		return arg, nil
	case int8:
		return int64(arg), nil
	case int16:
		return int64(arg), nil
	case int32:
		return int64(arg), nil
	case int64:
		return arg, nil
	case int:
		return int64(arg), nil
	case uint8:
		return int64(arg), nil
	case uint16:
		return int64(arg), nil
	case uint32:
		return int64(arg), nil
	case uint64:
		if arg > math.MaxInt64 {
			return nil, fmt.Errorf("arg too big for int64: %v", arg)
		}
		return int64(arg), nil
	case uint:
		if uint64(arg) > math.MaxInt64 {
			return nil, fmt.Errorf("arg too big for int64: %v", arg)
		}
		return int64(arg), nil
	}

	if dt, found := ci.DataTypeForValue(arg); found {
		v := dt.Value
		err := v.Set(arg)
		if err != nil {
			return nil, err
		}
		buf, err := v.(pgtype.TextEncoder).EncodeText(ci, nil)
		if err != nil {
			return nil, err
		}
		if buf == nil {
			return nil, nil
		}
		return string(buf), nil
	}

	if refVal.Kind() == reflect.Ptr {
		arg = refVal.Elem().Interface()
		return convertSimpleArgument(ci, arg)
	}

	if strippedArg, ok := stripNamedType(&refVal); ok {
		return convertSimpleArgument(ci, strippedArg)
	}
	return nil, SerializationError(fmt.Sprintf("Cannot encode %T in simple protocol - %T must implement driver.Valuer, pgtype.TextEncoder, or be a native type", arg, arg))
}

func encodePreparedStatementArgument(ci *pgtype.ConnInfo, buf []byte, oid uint32, arg interface{}) ([]byte, error) {
	if arg == nil {
		return pgio.AppendInt32(buf, -1), nil
	}

	switch arg := arg.(type) {
	case pgtype.BinaryEncoder:
		sp := len(buf)
		buf = pgio.AppendInt32(buf, -1)
		argBuf, err := arg.EncodeBinary(ci, buf)
		if err != nil {
			return nil, err
		}
		if argBuf != nil {
			buf = argBuf
			pgio.SetInt32(buf[sp:], int32(len(buf[sp:])-4))
		}
		return buf, nil
	case pgtype.TextEncoder:
		sp := len(buf)
		buf = pgio.AppendInt32(buf, -1)
		argBuf, err := arg.EncodeText(ci, buf)
		if err != nil {
			return nil, err
		}
		if argBuf != nil {
			buf = argBuf
			pgio.SetInt32(buf[sp:], int32(len(buf[sp:])-4))
		}
		return buf, nil
	case string:
		buf = pgio.AppendInt32(buf, int32(len(arg)))
		buf = append(buf, arg...)
		return buf, nil
	}

	refVal := reflect.ValueOf(arg)

	if refVal.Kind() == reflect.Ptr {
		if refVal.IsNil() {
			return pgio.AppendInt32(buf, -1), nil
		}
		arg = refVal.Elem().Interface()
		return encodePreparedStatementArgument(ci, buf, oid, arg)
	}

	if dt, ok := ci.DataTypeForOID(oid); ok {
		value := dt.Value
		err := value.Set(arg)
		if err != nil {
			{
				if arg, ok := arg.(driver.Valuer); ok {
					v, err := callValuerValue(arg)
					if err != nil {
						return nil, err
					}
					return encodePreparedStatementArgument(ci, buf, oid, v)
				}
			}

			return nil, err
		}

		sp := len(buf)
		buf = pgio.AppendInt32(buf, -1)
		argBuf, err := value.(pgtype.BinaryEncoder).EncodeBinary(ci, buf)
		if err != nil {
			return nil, err
		}
		if argBuf != nil {
			buf = argBuf
			pgio.SetInt32(buf[sp:], int32(len(buf[sp:])-4))
		}
		return buf, nil
	}

	if strippedArg, ok := stripNamedType(&refVal); ok {
		return encodePreparedStatementArgument(ci, buf, oid, strippedArg)
	}
	return nil, SerializationError(fmt.Sprintf("Cannot encode %T into oid %v - %T must implement Encoder or be converted to a string", arg, oid, arg))
}

chooseParameterFormatCode determines the correct format code for an argument to a prepared statement. It defaults to TextFormatCode if no determination can be made.

func chooseParameterFormatCode(ci *pgtype.ConnInfo, oid uint32, arg interface{}) int16 {
	switch arg := arg.(type) {
	case pgtype.ParamFormatPreferrer:
		return arg.PreferredParamFormat()
	case pgtype.BinaryEncoder:
		return BinaryFormatCode
	case string, *string, pgtype.TextEncoder:
		return TextFormatCode
	}

	return ci.ParamFormatCodeForOID(oid)
}

func stripNamedType(val *reflect.Value) (interface{}, bool) {
	switch val.Kind() {
	case reflect.Int:
		convVal := int(val.Int())
		return convVal, reflect.TypeOf(convVal) != val.Type()
	case reflect.Int8:
		convVal := int8(val.Int())
		return convVal, reflect.TypeOf(convVal) != val.Type()
	case reflect.Int16:
		convVal := int16(val.Int())
		return convVal, reflect.TypeOf(convVal) != val.Type()
	case reflect.Int32:
		convVal := int32(val.Int())
		return convVal, reflect.TypeOf(convVal) != val.Type()
	case reflect.Int64:
		convVal := int64(val.Int())
		return convVal, reflect.TypeOf(convVal) != val.Type()
	case reflect.Uint:
		convVal := uint(val.Uint())
		return convVal, reflect.TypeOf(convVal) != val.Type()
	case reflect.Uint8:
		convVal := uint8(val.Uint())
		return convVal, reflect.TypeOf(convVal) != val.Type()
	case reflect.Uint16:
		convVal := uint16(val.Uint())
		return convVal, reflect.TypeOf(convVal) != val.Type()
	case reflect.Uint32:
		convVal := uint32(val.Uint())
		return convVal, reflect.TypeOf(convVal) != val.Type()
	case reflect.Uint64:
		convVal := uint64(val.Uint())
		return convVal, reflect.TypeOf(convVal) != val.Type()
	case reflect.String:
		convVal := val.String()
		return convVal, reflect.TypeOf(convVal) != val.Type()
	}

	return nil, false