Executing SQL query with variable number of named parameters in Golang

Question

So I have this PostgreSQL function, which takes variable number of named arguments and returns list of corresponding items:

CREATE OR REPLACE FUNCTION read_user(
  _id BIGINT DEFAULT NULL,
  _phone VARCHAR(30) DEFAULT NULL,
  _type VARCHAR(15) DEFAULT NULL,
  _last VARCHAR(50) DEFAULT NULL,
  _first VARCHAR(50) DEFAULT NULL
) 
RETURNS setof T_USERS
AS $$ 
BEGIN
  RETURN QUERY
  SELECT * FROM T_USERS
  WHERE ( id = _id OR _id IS NULL )
    AND ( phone = _phone OR _phone IS NULL )
    AND ( type = _type OR _type IS NULL )
    AND ( last = _last OR _last IS NULL )
    AND ( first = _first OR _first IS NULL );
    EXCEPTION WHEN others THEN
      RAISE WARNING 'Transaction failed and was rolled back';
      RAISE NOTICE '% %', SQLERRM, SQLSTATE;
END
$$ LANGUAGE plpgsql;

So I can run polymorphic queries like these:

SELECT read_user(_id := 2);
SELECT read_user(_first := 'John', _last := 'Doe');

In Golang I can make something like:

stmt, err := db.Prepare("SELECT read_user(_id = ?)")

But how can I do the same, but with variable amount of read_user arguments? I'm using pq driver https://github.com/lib/pq.

What db driver are you using to talk to postgres?

mkopriva
– mkopriva

2017-04-29 21:35:04 +00:00
Commented Apr 29, 2017 at 21:35 — mkopriva
– mkopriva, Commented Apr 29, 2017 at 21:35
@mkopriva link pq

Yakkodash
– Yakkodash

2017-04-29 21:40:45 +00:00
Commented Apr 29, 2017 at 21:40 — Yakkodash
– Yakkodash, Commented Apr 29, 2017 at 21:40

mkopriva · Accepted Answer · 2017-04-29 22:38:25Z

4

You can construct your one statement by enumerating all the parameters with their placeholders and then you could pass nil explicitly where you don't have the parameter value.

stmt, err := db.Prepare("SELECT read_user(_id := $1, _phone := $2, _type := $3, _last := $4, _first := $5)")
if err != nil {
    // ...
}
stmt.Query(2, nil, nil, nil, nil) // result should be equivalent to `SELECT read_user(_id := 2)`
stmt.Query(nil, nil, nil, "Doe", "John") // result should be equivalent to `SELECT read_user(_first := 'John', _last := 'Doe')`

And if you want to have named parameters in Go as well, you can create a struct type to represent the parameters and a wrapper func that'll map that parameter type's fields into the query:

type readUserParams struct {
    Id    interface{}
    Phone interface{}
    Type  interface{}
    Last  interface{}
    First interface{}
}

func readUser(p *readUserParams) {
    stmt.Query(p.Id, p.Phone, p.Type, p.Last, p.First)
    // ...
}

readUser(&readUserParams{Id: 2})
readUser(&readUserParams{First: "John", Last:"Doe"})

edited Apr 29, 2017 at 22:38

answered Apr 29, 2017 at 22:09

mkopriva

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4 Comments

Yakkodash Over a year ago

It works, but could you explain how nil is translated to named parameter in postgres? I expected postgres throw some kind of error considering assigning parameter to type, which is not expected.

mkopriva Over a year ago

I believe nil gets translated into NULL and calling the postgres function like this SELECT read_user(_id := 2, _phone := NULL, _type := NULL...) is valid.

Pat Over a year ago

This solution also works with the snowflake implementation of go :) godoc.org/github.com/snowflakedb/gosnowflake

sofs1 Over a year ago

@Pat - Your comment resolved my issue after 7 hours. Thanks.

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