Module hdbconnect::code_examples

Code examples.

1. Database connections

Establish an authenticated connection to the database server (see also ConnectParams):

use hdbconnect::{Connection, IntoConnectParams};
// connect directly to a database:
let mut connection1 = Connection::new("hdbsql://my_user:my_passwd@the_host:30815")?;

// connect to the port of the system db and let the db redirect to the correct host and port:
let mut connection2 = Connection::new("hdbsql://my_user:my_passwd@the_host:30813?db=MEI")?;

2. Queries and other database calls

2.1 Generic method: Connection::statement() and HdbResponse

The most generic way to fire SQL statements without preparation is using Connection::statement(). This generic method can handle very different kinds of calls (SQL queries, DML, procedure calls), and thus has the most generic OK return type, HdbResponse.

let query = "SELECT foo FROM bar";
let response = connection.statement(query)?; // HdbResponse

HdbResponse covers all possible return values we can get from the database. You thus have to analyze it to get to the concrete response to your call. Or you use the respective short-cut method that fits to your statement.

let response = connection.statement(query)?; // HdbResponse
let resultset = response.into_resultset()?; // ResultSet

You can do the same of course with HdbResponses obtained from the execution of prepared statements.

2.2 More specific methods with tailored return values

In many cases it will be more appropriate and convenient to send your database command with one of the more specialized methods

• Connection::query() // HdbResult<ResultSet>
• Connection::dml() // HdbResult<usize>
• Connection::exec() // HdbResult<()>

which convert the database response directly into a simpler result type:

let qry = "SELECT foo FROM bar";
let resultset = connection.query(qry)?; // ResultSet

2.3 Prepared statements

With prepared statements you can use parameters in a database statement, and provide one or more sets of these parameters in separate API calls before executing the statement. A parameter set is provided as a reference to a rust value that implements serde's Serialize, and the serialized field structure must be convertible into the expected parameter value types.

Using a prepared statement could look like this:

#[derive(Serialize)]
struct Values{
   s: &'static str,
   i: i32,
};
let v1 = Values{s: "foo", i:45};
let v2 = Values{s: "bar", i:46};

let mut stmt = connection.prepare("insert into COUNTERS (S_KEY, I_VALUE) values(?, ?)")?;
stmt.add_batch(&v1)?;
stmt.add_batch(&v2)?;
stmt.execute_batch()?;

Or like this:

let mut stmt = connection.prepare("select NAME, CITY from PEOPLE where iq > ? and age > ?")?;
stmt.add_batch(&(100_u8, 45_i32))?;
let resultset = stmt.execute_batch()?.into_resultset()?;

3. Result set evaluation

Some of the following examples use the method try_into(), on an individual HdbValue, a Row, or a ResultSet. These methods are based on the deserialization part of serde and use return type polymorphism, which means that you need to specify explicitly the desired type of the return value.

3.1 Iterating over rows

Evaluating a result set by iterating over the rows explicitly is possible, of course. Note that the row iterator returns HdbResult<Row>, not Row, because the result set might need to fetch more rows lazily from the database, which can fail.

for row in resultset {
    let row = row?;
    // now you have a real row
}

Such a streaming-like behavior is especially appropriate for large result sets. Iterating over the rows, while they are fetched on-demand from the server in smaller portions, makes it easy to write complex evaluations in an efficient and scalable manner.

let key_figure = resultset.map(|r|{r.unwrap()}).map(...).fold(...).collect(...);

3.2 Explicitly evaluating a single row

You can retrieve the field values of a row individually, one after the other:

for row in resultset {
    let mut row:Row = row?;
    let f1: String = row.next_try_into()?;
    let f2: Option<i32> = row.next_try_into()?;
    let f3: i32 = row.next_try_into()?;
    let f4: chrono::NaiveDateTime = row.next_try_into()?;
}

3.3 Direct conversion of entire rows

A usually more convenient way is to convert the complete row into a normal rust value or tuple or struct:

#[derive(Deserialize)]
struct TestData {/* ...*/};
let qry = "select * from TEST_RESULTSET";
for row in connection.query(qry)? {
    let td: TestData = row?.try_into()?;
}

3.4 Direct conversion of entire result sets

Even more convenient is the option to convert the complete result set in a single step. Depending on the concrete numbers of rows and columns, this option supports a variety of target data structures.

3.4.1 Matrix-structured result sets

You can always, and most often want to, use a Vec of a struct or tuple that matches the fields of the result set.

#[derive(Deserialize)]
struct MyRow {/* ...*/}

let result: Vec<MyRow> = connection.query(qry)?.try_into()?;

3.4.2 Single-line result sets

If the result set contains only a single line (e.g. because you specified TOP 1 in your select, or you qualified the full primary key), then you can also deserialize directly into a plain MyRow.

let result: MyRow = connection.query(qry)?.try_into()?;

3.4.3 Single-column result sets

If the result set contains only a single column, then you can choose to deserialize into a Vec<field>, where field is a type that matches the field of the result set.

let result: Vec<u32> = connection.query(qry)?.try_into()?;

3.4.4 Single-value result sets

If the result set contains only a single value (one row with one column), then you can also deserialize into a plain field:

let result: u32 = connection.query(qry)?.try_into()?;

4. Deserialization of field values

The deserialization of individual values provides flexibility without data loss:

• You can e.g. convert values from a nullable column into a plain field, provided that no NULL values are given in the result set.

• Vice versa, you can use an Option<field> as target structure, even if the column is marked as NOT NULL.

• Source and target integer types can differ from each other, as long as the concrete values can be assigned without loss.

• You can convert numeric values on-the-fly into default String representations.

You should use this flexibility with some care though, or you will face errors when the data violates the boundaries of the target values.

5. Binary Values

So far, specialization support is not yet in rust stable. Without that, you have to use serde_bytes::Bytes and serde_bytes::ByteBuf as lean wrappers around &[u8] and Vec<u8> to serialize into or deserialize from binary database types.

let raw_data: Vec<u8> = ...;
insert_stmt.add_batch(&(Bytes::new(&*raw_data)))?;

let bindata: serde_bytes::ByteBuf = resultset.try_into()?; // single binary field
let first_byte = bindata[0];

6. LOBs

Binary and Character LOBs can be treated like "normal" binary and String data, i.e. you can convert them with the methods described above into serde_bytes::ByteBuf or String values (see serde_bytes for serde's specialties regarding bytes).

If necessary, you can easily avoid materializing the complete "Large Object", and stream it e.g. into a writer. For doing so, you convert the value into one of hdbconnect::{BLob, CLob, NCLob}.

In this example the NCLob will, while being read by std::io::copy(), continuously fetch more data from the database until it is completely transferred:

use hdbconnect::{Connection, HdbResult, IntoConnectParams, ResultSet};
use hdbconnect::types::NCLob;
let mut nclob: NCLob = resultset.into_single_row()?.into_single_value()?.try_into_nclob()?;
std::io::copy(&mut nclob, &mut writer)?;