Add an example to expose the new possibilities of no more RwTxn

heed/examples/non-mutable-rwtxn.rs

@@ -0,0 +1,97 @@
+use std::error::Error;
+use std::fs;
+use std::path::Path;
+
+use heed::types::*;
+use heed::{Database, EnvOpenOptions};
+
+/// This example exposes some of the possibilities using
+/// a non-mutable RwTxn to iterate on a database permits.
+fn main() -> Result<(), Box<dyn Error>> {
+    let path = Path::new("target").join("heed.mdb");
+
+    fs::create_dir_all(&path)?;
+
+    let env = EnvOpenOptions::new()
+        .map_size(10 * 1024 * 1024) // 10MB
+        .max_dbs(3000)
+        .open(path)?;
+
+    // here the key will be an str and the data will be a slice of u8
+    let mut wtxn = env.write_txn()?;
+    let one: Database<Str, ByteSlice> = env.create_database(&mut wtxn, Some("one"))?;
+    let two: Database<Str, ByteSlice> = env.create_database(&mut wtxn, Some("two"))?;
+
+    // clear db
+    one.clear(&wtxn)?;
+    two.clear(&wtxn)?;
+    wtxn.commit()?;
+
+    // -----
+
+    let wtxn = env.write_txn()?;
+    for number in 0u32..1000 {
+        let k = number.to_string();
+        let v = number.to_be_bytes();
+        one.put(&wtxn, &k, &v)?;
+    }
+
+    for result in one.iter(&wtxn)? {
+        let (k, v) = result?;
+        two.put(&wtxn, k, v)?;
+    }
+
+    for (res1, res2) in one.iter(&wtxn)?.zip(two.iter(&wtxn)?) {
+        let (k1, v1) = res1?;
+        let (k2, v2) = res2?;
+        assert_eq!(k1, k2);
+        assert_eq!(v1, v2);
+    }
+
+    wtxn.commit()?;
+
+    // -----
+
+    let wtxn = env.write_txn()?;
+    for result in one.iter(&wtxn)? {
+        let (k, v) = result?;
+        if k == "10" {
+            let n: u32 = 11;
+            one.put(&wtxn, &n.to_string(), &[])?;
+        }
+        if k == "11" {
+            assert!(v.is_empty());
+        }
+    }
+
+    wtxn.commit()?;
+
+    // -----
+
+    let wtxn = env.write_txn()?;
+    let n: u32 = 100_000;
+    one.put(&wtxn, &n.to_string(), &n.to_be_bytes())?;
+    let v = one.get(&wtxn, &n.to_string())?.unwrap();
+    one.put(&wtxn, &n.to_string(), v)?;
+
+    let v = one.get(&wtxn, &n.to_string())?.unwrap();
+    assert_eq!(v, n.to_be_bytes());
+
+    wtxn.commit()?;
+
+    // -----
+
+    // What happen when we clear the database while iterating on it?
+    let wtxn = env.write_txn()?;
+    for result in one.remap_data_type::<DecodeIgnore>().iter(&wtxn)? {
+        let (k, _) = result?;
+        if k == "10" {
+            one.clear(&wtxn)?;
+        }
+    }
+
+    assert!(one.is_empty(&wtxn)?);
+    wtxn.commit()?;
+
+    Ok(())
+}