Add postgres examples

examples/examples/seed_postgres_db.rs

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+use tokio_postgres::{Error, GenericClient, NoTls};
+
+/// docker run --name postgres -e POSTGRES_PASSWORD=password -e POSTGRES_DB=postgres_db -p 5432:5432 -d postgres:16-alpine
+#[tokio::main]
+async fn main() -> Result<(), Error> {
+    let (client, connection) = tokio_postgres::connect(
+        "host=localhost user=postgres password=password dbname=postgres_db",
+        NoTls,
+    )
+    .await?;
+
+    // The connection object performs the actual communication with the database,
+    // so spawn it off to run on its own.
+    tokio::spawn(async move {
+        if let Err(e) = connection.await {
+            eprintln!("connection error: {}", e);
+        }
+    });
+
+    let table_name = "companies";
+    client
+        .execute(
+            format!(
+                r#"CREATE TABLE IF NOT EXISTS {} (id SERIAL PRIMARY KEY, name VARCHAR)"#,
+                table_name
+            )
+            .as_str(),
+            &[],
+        )
+        .await?;
+
+    let stmt = client
+        .prepare(format!("INSERT INTO {} (name) VALUES ($1)", table_name).as_str())
+        .await?;
+    client.execute(&stmt, &[&"test"]).await?;
+
+    Ok(())
+}

examples/examples/sink_to_postgres.rs

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+use std::time::Duration;
+
+use datafusion::error::Result;
+use datafusion::functions_aggregate::average::avg;
+use datafusion::logical_expr::{col, max, min};
+
+use df_streams_core::context::Context;
+use df_streams_core::datasource::kafka::{ConnectionOpts, KafkaTopicBuilder};
+use df_streams_core::physical_plan::utils::time::TimestampUnit;
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    let sample_event = r#"{"occurred_at_ms": 1715201766763, "temperature": 87.2}"#;
+
+    let bootstrap_servers = String::from("localhost:9092");
+
+    let ctx = Context::new()?;
+
+    let mut topic_builder = KafkaTopicBuilder::new(bootstrap_servers.clone());
+
+    let source_topic = topic_builder
+        .with_timestamp(String::from("occurred_at_ms"), TimestampUnit::Int64Millis)
+        .with_encoding("json")?
+        .with_topic(String::from("temperature"))
+        .infer_schema_from_json(sample_event)?
+        .build_reader(ConnectionOpts::from([
+            ("auto.offset.reset".to_string(), "earliest".to_string()),
+            ("group.id".to_string(), "sample_pipeline".to_string()),
+        ]))
+        .await?;
+
+    let ds = ctx.from_topic(source_topic).await?.streaming_window(
+        vec![],
+        vec![
+            min(col("temperature")).alias("min"),
+            max(col("temperature")).alias("max"),
+            avg(col("temperature")).alias("average"),
+        ],
+        Duration::from_millis(1_000), // 5 second window
+        None,
+    )?;
+
+    // ds.clone().print_stream().await?;
+
+    println!("{}", ds.df.schema());
+
+    Ok(())
+}