`pgx.Conn.Query()` behavior for JSON string value of "null" returns incorrect value

[zachmu]

This query incorrectly returns a nil value when using the pgx.Conn.Query() method:

select 'null'::json;

This response is the JSON string literal value "null", not the JSON null literal. The JSON codec is incorrectly parsing this response into a nil value.

Here's a simple repro:

package main

import (
	"context"
	"fmt"
	"log"

	"github.com/jackc/pgx/v5"
)

func main() {
	// Adjust connection string as needed
	connStr := "postgres://postgres:postgres@localhost:5432/postgres"

	conn, err := pgx.Connect(context.Background(), connStr)
	if err != nil {
		log.Fatal("connect error:", err)
	}
	defer conn.Close(context.Background())

	rows, err := conn.Query(context.Background(), `select 'null'::json;`)
	if err != nil {
		log.Fatal("query error:", err)
	}

	var rowSlice []any
	for rows.Next() {
		row, err := rows.Values()
		if err != nil {
			log.Fatal("query error:", err)
		}
		rowSlice = append(rowSlice, row)
	}

	fmt.Println("Result:", rowSlice)
}

Prints:

Result: [[<nil>]]

This works correctly using the QueryRow() and Scan() interfaces directly with a string scan target, like this:

	var result string

	err = conn.QueryRow(context.Background(), `select 'null'::json;`).Scan(&result)

Also works correctly with the vanilla postgres connector:

package main

import (
	"database/sql"
	"fmt"
	"log"

	_ "github.com/lib/pq"
)

func main() {
	connStr := "host=localhost port=5432 user=postgres password=postgres dbname=postgres sslmode=disable"

	db, err := sql.Open("postgres", connStr)
	if err != nil {
		log.Fatal(err)
	}
	defer db.Close()

	var result string

	err = db.QueryRow(`select 'null'::json;`).Scan(&result)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println("Result:", result)
}

Prints:

Result: null

Version

• Go: go version go1.25.3 darwin/arm64
• PostgreSQL: PostgreSQL 18.0 (Homebrew) on aarch64-apple-darwin24.4.0, compiled by Apple clang version 17.0.0 (clang-1700.0.13.3), 64-bit
• pgx: v5.6.1-0.20240826124046-97d20ccfadaa

[jackc]

I believe the current behavior is correct, though a bit confusing. The issue is how is PostgreSQL json scanned into a Go string. In pgx, scanning into a string copies the json text. This is what you expected. But Values() is the equivalent of scanning into an any. A json null converts to nil any. If this was any other way you wouldn't be able to distinguish between a json null and a json "null".

See below:

package main

import (
	"context"
	"fmt"
	"log"
	"os"

	"github.com/jackc/pgx/v5"
)

func main() {
	ctx := context.Background()

	conn, err := pgx.Connect(ctx, os.Getenv("DATABASE_URL"))
	if err != nil {
		log.Fatal(err)
	}
	defer conn.Close(ctx)

	var v1, v2 any
	var s string
	err = conn.QueryRow(ctx, `select 'null'::json, '"null"'::json, '{"foo":"bar"}'::json`).Scan(&v1, &v2, &s)
	if err != nil {
		log.Fatal("query error:", err)
	}

	fmt.Printf("Result: %#v %#v %#v\n", v1, v2, s)
}

Result: <nil> "null" "{\"foo\":\"bar\"}"