lazy_query example

[stepankuzmin]

Hi there! I want to stream a large set of rows from a database. I didn't find any examples of lazy_query and tried to do it like this:

let query = format!("select row_to_json({1}) from {0}.{1}", schema, table);

let trans = conn.transaction().unwrap();
let stmt = trans.prepare(&query).unwrap();
let result = stmt.lazy_query(&trans, &[], 2);

for rows in result {
    println!("row: {:?}", rows);
}

I'm getting LazyRows { name: "s0p0", row_limit: 2, remaining_rows: 2, more_rows: true } as result. So how can I get rows from LazyRows?

[sfackler]

You're missing an .unwrap() on the lazy_query call. Rather than iterating over the rows, you're iterating over the Ok value of the Result<LazyRows, Error>.

[stepankuzmin]

Thanks for reply, @sfackler! I've tried

let result = stmt.lazy_query(&trans, &[], 2).unwrap();

for row in result {
    println!("row: {:?}", row);
}

But the compiler says, that the trait Iterator is not implemented for LazyRows:

41 | /     for row in result {
42 | |         println!("row: {:?}", row);
43 | |     }
   | |_____^ `postgres::rows::LazyRows<'_, '_>` is not an iterator; maybe try calling `.iter()` or a similar method
   |
   = help: the trait `std::iter::Iterator` is not implemented for `postgres::rows::LazyRows<'_, '_>`
   = note: required by `std::iter::IntoIterator::into_iter`

P.S. I'm using

postgres = { version = "0.15.1", features = ["with-serde_json", "with-time", "with-uuid"] }

[sfackler]

Ah right - the other bit here is that LazyRows implements the FallibleIterator rather than the normal Iterator trait.

This should do what you want:

extern crate fallible_iterator;

use fallible_iterator::FallibleIterator;

let trans = conn.transaction().unwrap();
let stmt = trans.prepare(&query).unwrap();
let mut result = stmt.lazy_query(&trans, &[], 2).unwrap();

while let Some(row) = result.next().unwrap() {
    println!("row: {:?}", row);
}

[sfackler]

I've added an example to the documentation to make this less confusing: 89d39cc.

[stepankuzmin]

Great! Thanks!

[stepankuzmin]

One more question: In the example above I'm querying 4k rows, but the body of while loop is only called once. It seems, that row_limit does not affect query.

[sfackler]

The row limit doesn't affect the user-visible access to the rows, it just controls the batch size that those rows are loaded from the database. Loading rows in batches of 1 or batches of 4K will behave the same way when looping (other than 1 row per batch probably being a lost slower!).

[stepankuzmin]

Wow, thanks! That's clear for me now.