对于generate_series的Postgres查询中的占位符返回未知

时间:2015-08-18 02:22:53

标签: postgresql go generate-series

我尝试使用像这样的占位符进行查询

database.Query("select login,displayname from (select distinct $1+trunc(random()*$2)::integer as id from generate_series($3,$4) g ) r join users using(id) limit 10",min_id,max_id-min_id,min_id,max_id)

它给我一个错误:

pq: function generate_series(unknown, unknown) is not unique

然后我找到了将查询格式化为字符串

的解决方案
query:=fmt.Sprintf("select login,displayname from (select distinct %v+trunc(random()*%v)::integer as id from generate_series(%v,%v) g ) r join users using(id) limit 10",min_id,max_id-min_id,min_id,max_id)

它有效。

我想让它以正确的方式运作,但我不明白为什么它没有。

更新:

var min_id int64
var max_id int64
err:=_database.QueryRow("select min(id),max(id) from users").Scan(&min_id, &max_id)
if err!=nil { 
    log.Panicf("Failed to get min and max %v",err.Error())
    return
}

var rows *sql.Rows

query:=fmt.Sprintf("select login,displayname from (select distinct %v+trunc(random()*%v)::integer as id from generate_series(%v,%v) g ) r join users using(id) limit 10",min_id,max_id-min_id,min_id,max_id)
log.Printf(query)
rows,err=_database.Query("select login,displayname from (select distinct $1+trunc(random()*$2)::integer as id from generate_series($3,$4) g ) r join users using(id) limit 10",min_id,max_id-min_id,min_id,max_id)
if err!=nil {
    log.Panicf("failed to get random entries: %v",err)
}

1 个答案:

答案 0 :(得分:7)

尝试分开查询。你会看到第一部分工作正常:

db.QueryRow("SELECT $1+trunc(random()*$2) as test", 10, 5)

这可能有效,因为$1$2与数学运算符一起使用(和/或因为trunc()random()都返回数字),所以postgres可以推断数据类型。

所以看起来它是generate_series()参数没有被正确确定。 Postgres 可以根据参数推断数据类型,例如

if err := db.QueryRow("SELECT trunc($1,2) as test", 1.4343).Scan(&output); err != nil {panic(err)}
// output = 1.43

然而,如果它是一个多态函数,就会出现歧义,它会失败,例如。

if err := db.QueryRow("SELECT trunc($1) as test", 1.4343).Scan(&output); err != nil {panic(err)}
// panic: pq: function trunc(unknown) is not unique

为避免歧义,请在预准备语句中显式转换参数,例如:generate_series($3::int,$4::int)