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Rows.Scan fails with Scanner objects, but Row.Scan works ok with Scanner objects. #1505
Description
Issue description
If I use scanner objects with Rows.Scan, the src argument is a []uint8 for all types.
Example code
My table is
CREATE TABLE IF NOT EXISTS fruits (
id BIGINT(20) NOT NULL AUTO_INCREMENT,
name VARCHAR(45),
year SMALLINT,
amount INT,
price FLOAT(23),
PRIMARY KEY (id)
) ENGINE=InnoDB CHARSET=utf8mb4;Here is my code
type Scanner struct {
FieldName string
}
func (s *Scanner) Scan(src any) error {
fmt.Printf("FieldName: %s, src: type=%[2]T value=%[2]v", s.FieldName, src)
return nil
}
// prepare to query
id_scanner := &Scanner{"Id"}
name_scanner := &Scanner{"Name"}
year_scanner := &Scanner{"Year"}
amount_scanner := &Scanner{"Amount"}
price_scanner := &Scanner{"Price"}
// query single row
row := db.QueryRow("select * from fruits where id=?", 1)
err := row.Scan(id_scanner, name_scanner, year_scanner, amount_scanner, price_scanner)The err is nil and the output looks very nice, such as
FieldName: Id, src: type=int64 value=1
FieldName: Name, src: type=[]uint8 value=[97 112 112 108 101]
FieldName: Year, src: type=int64 value=1965
FieldName: Amount, src: type=int64 value=123
FieldName: Price, src: type=float32 value=7.55
This shows that Row.Scan works with Scanner objects. However, Rows.Scan fails:
rows, err := db.Query("select * from fruits;")
// err is nil
defer rows.Close()
for rows.Next() {
err := rows.Scan(id_scanner, name_scanner, year_scanner, amount_scanner, price_scanner)
// err is nil, but output is wrong
}Error log
The output is different:
FieldName: Id, src: type=[]uint8 value=[49]
FieldName: Name, src: type=[]uint8 value=[97 112 112 108 101]
FieldName: Year, src: type=[]uint8 value=[49 57 54 53]
FieldName: Amount, src: type=[]uint8 value=[49 50 51]
FieldName: Price, src: type=[]uint8 value=[55 46 53 53]
For the Id field, the value 49 is the ASCII '1'.
For the Name field, the value [97 112 112 108 101] is ASCII apple.
For the Year field, the value [49 57 54 53] is ASCII 1965.
For the Amount field, the value [49 50 51] is ASCII 123.
For the Price field, the value [55 46 53 53] is ASCII 7.55.
It looks like the src argument is the result of fmt.Print or something similar if Scanner objects are involved.
Configuration
This Scanner code works with Microsoft SQL Server. But it doesn't work with MySQL and I am testing the Scanner code against the mysql:8..0.31 docker image with go-1.20 on archlinux. My go.mod shows I am using github.com/go-sql-driver/mysql v1.7.1.
Activity
Fung-Chai commentedon Oct 25, 2023
Using the
delvedebugger, I dug into thego-sql-drivercode and discovered thatRow.ScancallsbinaryRows.NextandRows.ScancallstextRows.Next. This explains whyQueryRowreturns the correct types butQueryreturns[]uint8for all columns. The question now is whyRows.Scandoesn't callbinaryRows.Nextas well. I will dig on.Fung-Chai commentedon Oct 25, 2023
Another discovery.
My original code, which makes use of
Queryproduces the wrong results.But I can get the right output with
Prepare, callingQueryvia the statement object.The
mysqlConn.queyyreturnstextRowswhilemysqlStmt.queryreturns *binaryRows.methane commentedon Oct 25, 2023
Using placeholder makes your query binary.
When implementing Scanner, you should expect many
driver.Valuetype input, not single consistent one.See #1452 for mitigations implemented in next release.