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# Apache DataFusion SQL Query Planner

This crate provides a general purpose SQL query planner that can parse SQL and translate queries into logical
plans. Although this crate is used by the [Apache DataFusion] query engine, it was designed to be easily usable from any
project that requires a SQL query planner and does not make any assumptions about how the resulting logical plan
will be translated to a physical plan. For example, there is no concept of row-based versus columnar execution in the
logical plan.

Note that the [`datafusion`] crate re-exports this module. If you are already
using the [`datafusion`] crate in your project, there is no reason to use this
crate directly in your project as well.

[apache datafusion]: https://datafusion.apache.org/
[`datafusion`]: https://crates.io/crates/datafusion

## Example Usage

See the [examples] directory for fully working examples.

Here is an example of producing a logical plan from a SQL string.

```rust,ignore
fn main() {
    let sql = "SELECT \
            c.id, c.first_name, c.last_name, \
            COUNT(*) as num_orders, \
            SUM(o.price) AS total_price, \
            SUM(o.price * s.sales_tax) AS state_tax \
        FROM customer c \
        JOIN state s ON c.state = s.id \
        JOIN orders o ON c.id = o.customer_id \
        WHERE o.price > 0 \
        AND c.last_name LIKE 'G%' \
        GROUP BY 1, 2, 3 \
        ORDER BY state_tax DESC";

    // parse the SQL
    let dialect = GenericDialect {}; // or AnsiDialect, or your own dialect ...
    let ast = Parser::parse_sql(&dialect, sql).unwrap();
    let statement = &ast[0];

    // create a logical query plan
    let context_provider = MyContextProvider::new();
    let sql_to_rel = SqlToRel::new(&context_provider);
    let plan = sql_to_rel.sql_statement_to_plan(statement.clone()).unwrap();

    // show the plan
    println!("{:?}", plan);
}
```

This is the logical plan that is produced from this example. Note that this is an **unoptimized**
logical plan. The [datafusion-optimizer] crate provides a query optimizer that can be applied to
plans produced by this crate.

```
Sort: state_tax DESC NULLS FIRST
  Projection: c.id, c.first_name, c.last_name, COUNT(Int64(1)) AS num_orders, SUM(o.price) AS total_price, SUM(o.price * s.sales_tax) AS state_tax
    Aggregate: groupBy=[[c.id, c.first_name, c.last_name]], aggr=[[COUNT(Int64(1)), SUM(o.price), SUM(o.price * s.sales_tax)]]
      Filter: o.price > Int64(0) AND c.last_name LIKE Utf8("G%")
        Inner Join: c.id = o.customer_id
          Inner Join: c.state = s.id
            SubqueryAlias: c
              TableScan: customer
            SubqueryAlias: s
              TableScan: state
          SubqueryAlias: o
            TableScan: orders
```

[examples]: examples
[datafusion-optimizer]: https://crates.io/crates/datafusion-optimizer
