feat: Implement delete for the storage engine (#777)

* docs: Fix incorrect comment of Vector::only_null

* feat: Add delete to WriteRequest and WriteBatch

* feat: Filter deleted rows

* fix: Fix panic after reopening engine

This is detected by adding a reopen step to the delete test for region.

* fix: Fix OpType::min_type()

* test: Add delete absent key test

* chore: Address CR comments

src/store-api/src/storage/requests.rs

@@ -31,6 +31,11 @@ pub trait WriteRequest: Send {
     ///
     /// `data` is the columnar format of the data to put.
     fn put(&mut self, data: HashMap<String, VectorRef>) -> Result<(), Self::Error>;
+
+    /// Delete rows by `keys`.
+    ///
+    /// `keys` are the row keys, in columnar format, of the rows to delete.
+    fn delete(&mut self, keys: HashMap<String, VectorRef>) -> Result<(), Self::Error>;
 }
 
 #[derive(Default)]

src/store-api/src/storage/types.rs

@@ -19,20 +19,27 @@
 pub type SequenceNumber = u64;
 
 /// Operation type of the value to write to storage.
+///
+/// The enum values are stored in the SST files so don't change
+/// them if possible.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub enum OpType {
+    /// Delete operation.
+    Delete = 0,
     /// Put operation.
-    Put,
+    Put = 1,
 }
 
 impl OpType {
+    /// Cast the [OpType] to u8.
+    #[inline]
     pub fn as_u8(&self) -> u8 {
         *self as u8
     }
 
     /// Minimal op type after casting to u8.
     pub const fn min_type() -> OpType {
-        OpType::Put
+        OpType::Delete
     }
 }
 
@@ -42,7 +49,8 @@ mod tests {
 
     #[test]
     fn test_op_type() {
-        assert_eq!(0, OpType::Put.as_u8());
+        assert_eq!(0, OpType::Delete.as_u8());
+        assert_eq!(1, OpType::Put.as_u8());
         assert_eq!(0, OpType::min_type().as_u8());
     }
 }