diff --git a/src/data/op.rs b/src/data/op.rs
index 9a0eb1cc..32f2650e 100644
--- a/src/data/op.rs
+++ b/src/data/op.rs
@@ -1,13 +1,41 @@
 use std::fmt::{Debug, Formatter};
+use crate::data::expr::Expr;
+use crate::data::typing::Typing;
 
 pub(crate) trait Op: Send + Sync {
     fn is_resolved(&self) -> bool;
     fn name(&self) -> &str;
+    fn non_null_args(&self) -> bool {
+        true
+    }
+    fn typing_eval(&self, args: ()) -> Typing {
+        Typing::Any
+    }
+    fn row_eval(&self, ctx: (), args: ()) -> () {
+        unimplemented!()
+    }
+    fn row_eval_non_null(&self) -> () {
+        panic!()
+    }
+    fn expr_eval(&self, ctx: (), args: ()) -> () {
+        self.row_eval(ctx, args)
+    }
 }
 
 pub(crate) trait AggOp: Send + Sync {
     fn is_resolved(&self) -> bool;
     fn name(&self) -> &str;
+    fn row_eval(&self, ctx: (), args: ()) -> () {
+        unimplemented!()
+    }
+    fn expr_eval(&self, ctx: (), args: ()) -> () {
+        self.row_eval(ctx, args)
+    }
+}
+
+impl<'a> Expr<'a> {
+    pub(crate) fn expr_eval() {}
+    pub(crate) fn row_eval() {}
 }
 
 pub(crate) struct UnresolvedOp(pub String);
diff --git a/src/data/tuple.rs b/src/data/tuple.rs
index f6ee6b50..b8ee3426 100644
--- a/src/data/tuple.rs
+++ b/src/data/tuple.rs
@@ -9,6 +9,7 @@ use std::hash::{Hash, Hasher};
 use std::result;
 use chrono::format::Item;
 use uuid::Uuid;
+use cozorocks::{PinnableSlicePtr, PinnableSlicePtrShared, SlicePtr, SlicePtrShared};
 
 #[derive(thiserror::Error, Debug)]
 pub enum TupleError {
@@ -426,7 +427,7 @@ impl<T: AsRef<[u8]>> Tuple<T> {
                 let (val, offset) = self.parse_value_at(pos + 1)?;
                 (offset, Value::DescVal(Reverse(val.into())))
             }
-            StorageTag::Max => (start, Value::EndSentinel),
+            StorageTag::Max => (start, Value::Sentinel),
         };
         Ok((val, nxt))
     }
@@ -534,6 +535,7 @@ impl OwnTuple {
     #[inline]
     pub(crate) fn seal_with_sentinel(&mut self) {
         self.push_tag(StorageTag::Max);
+        self.idx_cache.borrow_mut().push(self.data.len());
     }
     #[inline]
     fn push_tag(&mut self, tag: StorageTag) {
@@ -638,7 +640,7 @@ impl OwnTuple {
                 cache.truncate(start_len);
                 cache.push(self.data.len());
             }
-            Value::EndSentinel => panic!("Cannot push sentinel value"),
+            Value::Sentinel => self.seal_with_sentinel(),
             Value::DescVal(Reverse(v)) => {
                 self.push_reverse_value(v);
             }
@@ -727,4 +729,115 @@ impl<'a, P, T> From<(P, T)> for OwnTuple
         }
         ret
     }
-}
\ No newline at end of file
+}
+
+
+pub(crate) enum ReifiedTupleData {
+    Own(Vec<u8>),
+    Slice(SlicePtr),
+    SharedSlice(SlicePtrShared),
+    PinnableSlice(PinnableSlicePtr),
+    PinnableSliceShared(PinnableSlicePtrShared),
+}
+
+impl Clone for ReifiedTupleData {
+    fn clone(&self) -> Self {
+        match self {
+            ReifiedTupleData::Own(o) => ReifiedTupleData::Own(o.clone()),
+            ReifiedTupleData::Slice(s) => ReifiedTupleData::Own(s.as_ref().to_vec()),
+            ReifiedTupleData::SharedSlice(s) => ReifiedTupleData::SharedSlice(s.clone()),
+            ReifiedTupleData::PinnableSlice(s) => ReifiedTupleData::Own(s.as_ref().to_vec()),
+            ReifiedTupleData::PinnableSliceShared(s) => ReifiedTupleData::PinnableSliceShared(s.clone())
+        }
+    }
+}
+
+impl From<Vec<u8>> for ReifiedTupleData {
+    fn from(d: Vec<u8>) -> Self {
+        ReifiedTupleData::Own(d)
+    }
+}
+
+impl From<SlicePtr> for ReifiedTupleData {
+    fn from(d: SlicePtr) -> Self {
+        ReifiedTupleData::Slice(d)
+    }
+}
+
+impl From<SlicePtrShared> for ReifiedTupleData {
+    fn from(d: SlicePtrShared) -> Self {
+        ReifiedTupleData::SharedSlice(d)
+    }
+}
+
+impl From<PinnableSlicePtr> for ReifiedTupleData {
+    fn from(d: PinnableSlicePtr) -> Self {
+        ReifiedTupleData::PinnableSlice(d)
+    }
+}
+
+impl From<PinnableSlicePtrShared> for ReifiedTupleData {
+    fn from(d: PinnableSlicePtrShared) -> Self {
+        ReifiedTupleData::PinnableSliceShared(d)
+    }
+}
+
+
+impl AsRef<[u8]> for ReifiedTupleData {
+    fn as_ref(&self) -> &[u8] {
+        match self {
+            ReifiedTupleData::Own(o) => o.as_ref(),
+            ReifiedTupleData::Slice(s) => s.as_ref(),
+            ReifiedTupleData::SharedSlice(s) => s.as_ref(),
+            ReifiedTupleData::PinnableSlice(s) => s.as_ref(),
+            ReifiedTupleData::PinnableSliceShared(s) => s.as_ref()
+        }
+    }
+}
+
+pub(crate) type ReifiedTuple = Tuple<ReifiedTupleData>;
+
+impl From<OwnTuple> for ReifiedTuple {
+    fn from(t: OwnTuple) -> Self {
+        ReifiedTuple {
+            data: t.data.into(),
+            idx_cache: t.idx_cache,
+        }
+    }
+}
+
+impl From<Tuple<SlicePtr>> for ReifiedTuple {
+    fn from(t: Tuple<SlicePtr>) -> Self {
+        ReifiedTuple {
+            data: t.data.into(),
+            idx_cache: t.idx_cache,
+        }
+    }
+}
+
+impl From<Tuple<SlicePtrShared>> for ReifiedTuple {
+    fn from(t: Tuple<SlicePtrShared>) -> Self {
+        ReifiedTuple {
+            data: t.data.into(),
+            idx_cache: t.idx_cache,
+        }
+    }
+}
+
+impl From<Tuple<PinnableSlicePtr>> for ReifiedTuple {
+    fn from(t: Tuple<PinnableSlicePtr>) -> Self {
+        ReifiedTuple {
+            data: t.data.into(),
+            idx_cache: t.idx_cache,
+        }
+    }
+}
+
+impl From<Tuple<PinnableSlicePtrShared>> for ReifiedTuple {
+    fn from(t: Tuple<PinnableSlicePtrShared>) -> Self {
+        ReifiedTuple {
+            data: t.data.into(),
+            idx_cache: t.idx_cache,
+        }
+    }
+}
diff --git a/src/data/tuple_set.rs b/src/data/tuple_set.rs
index e0671431..d595505e 100644
--- a/src/data/tuple_set.rs
+++ b/src/data/tuple_set.rs
@@ -1,13 +1,21 @@
+use std::cmp::Ordering;
 use std::fmt::{Debug, Formatter};
 use std::result;
+use cozorocks::{PinnableSlicePtr, PinnableSlicePtrShared, SlicePtr, SlicePtrShared};
+use crate::data::tuple::{OwnTuple, ReifiedTuple, Tuple, TupleError};
+use crate::data::value::Value;
 
 #[derive(thiserror::Error, Debug)]
-pub(crate) enum TypingError {
+pub(crate) enum TupleSetError {
     #[error("table id not allowed: {0}")]
     InvalidTableId(u32),
+    #[error("Index out of bound: {0}")]
+    IndexOutOfBound(usize),
+    #[error(transparent)]
+    Tuple(#[from] TupleError),
 }
 
-type Result<T> = result::Result<T, TypingError>;
+type Result<T> = result::Result<T, TupleSetError>;
 
 pub(crate) const MIN_TABLE_ID_BOUND: u32 = 10000;
 
@@ -26,7 +34,7 @@ impl Debug for TableId {
 impl TableId {
     pub(crate) fn new(in_root: bool, id: u32) -> Result<Self> {
         if id <= MIN_TABLE_ID_BOUND {
-            Err(TypingError::InvalidTableId(id))
+            Err(TupleSetError::InvalidTableId(id))
         } else {
             Ok(TableId { in_root, id })
         }
@@ -78,3 +86,89 @@ impl Debug for TupleSetIdx {
         )
     }
 }
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub(crate) struct TupleSet {
+    keys: Vec<ReifiedTuple>,
+    vals: Vec<ReifiedTuple>,
+}
+
+impl TupleSet {
+    pub(crate) fn push_key(&mut self, t: ReifiedTuple) {
+        self.keys.push(t);
+    }
+    pub(crate) fn push_val(&mut self, v: ReifiedTuple) {
+        self.vals.push(v);
+    }
+    pub(crate) fn merge(&mut self, o: TupleSet) {
+        self.keys.extend(o.keys);
+        self.vals.extend(o.vals);
+    }
+    pub(crate) fn extend_keys<I, T>(&mut self, keys: I)
+        where I: IntoIterator<Item=T>,
+              ReifiedTuple: From<T> {
+        self.keys.extend(keys.into_iter().map(ReifiedTuple::from));
+    }
+    pub(crate) fn extend_vals<I, T>(&mut self, keys: I)
+        where I: IntoIterator<Item=T>,
+              ReifiedTuple: From<T> {
+        self.vals.extend(keys.into_iter().map(ReifiedTuple::from));
+    }
+
+    pub(crate) fn all_keys_eq(&self, other: &Self) -> bool {
+        if self.keys.len() != other.keys.len() {
+            return false;
+        }
+        for (l, r) in self.keys.iter().zip(&other.keys) {
+            if !l.key_part_eq(r) {
+                return false;
+            }
+        }
+        true
+    }
+    pub(crate) fn all_keys_cmp(&self, other: &Self) -> Ordering {
+        for (l, r) in self.keys.iter().zip(&other.keys) {
+            match l.key_part_cmp(r) {
+                Ordering::Equal => {}
+                v => return v,
+            }
+        }
+        Ordering::Equal
+    }
+
+    pub(crate) fn get_value(&self, TupleSetIdx { is_key, t_set, col_idx }: TupleSetIdx) -> Result<Value> {
+        let tuples = if is_key { &self.keys } else { &self.vals };
+        let tuple = tuples.get(t_set).ok_or(TupleSetError::IndexOutOfBound(t_set))?;
+        let res = tuple.get(col_idx)?;
+        Ok(res)
+    }
+}
+
+impl<I1, T1, I2, T2> From<(I1, I2)> for TupleSet
+    where I1: IntoIterator<Item=T1>,
+          ReifiedTuple: From<T1>,
+          I2: IntoIterator<Item=T2>,
+          ReifiedTuple: From<T2> {
+    fn from((keys, vals): (I1, I2)) -> Self {
+        TupleSet {
+            keys: keys.into_iter().map(ReifiedTuple::from).collect(),
+            vals: vals.into_iter().map(ReifiedTuple::from).collect(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::mem;
+    use super::*;
+
+    #[test]
+    fn sizes() {
+        let t = OwnTuple::with_prefix(0);
+        let t2 = OwnTuple::with_prefix(0);
+        let ts = TupleSet::from(([t], [t2]));
+        dbg!(ts);
+        dbg!(mem::size_of::<ReifiedTuple>());
+        dbg!(mem::size_of::<TupleSet>());
+    }
+}
\ No newline at end of file
diff --git a/src/data/typing.rs b/src/data/typing.rs
index 567ae6a1..a937520f 100644
--- a/src/data/typing.rs
+++ b/src/data/typing.rs
@@ -77,6 +77,9 @@ impl Debug for Typing {
 }
 
 impl Typing {
+    pub(crate) fn representative_value(&self) -> StaticValue {
+        todo!()
+    }
     pub(crate) fn coerce<'a>(&self, v: Value<'a>) -> Result<Value<'a>> {
         if *self == Typing::Any {
             return Ok(v);
diff --git a/src/data/value.rs b/src/data/value.rs
index b14e73de..8f91cc1b 100644
--- a/src/data/value.rs
+++ b/src/data/value.rs
@@ -20,7 +20,7 @@ pub enum Value<'a> {
 
     DescVal(Reverse<Box<Value<'a>>>),
 
-    EndSentinel,
+    Sentinel, // Acts as "any" in type inference, end value in sorting
 }
 
 pub(crate) type StaticValue = Value<'static>;
@@ -204,7 +204,7 @@ impl<'a> Display for Value<'a> {
                 }
                 f.write_char('}')?;
             }
-            Value::EndSentinel => write!(f, "Sentinel")?,
+            Value::Sentinel => write!(f, "Sentinel")?,
             Value::DescVal(Reverse(v)) => {
                 write!(f, "~{}", v)?;
             }
@@ -233,7 +233,7 @@ impl<'a> Value<'a> {
                 .map(|(k, v)| (Cow::Owned(k.into_owned()), v.to_static()))
                 .collect::<BTreeMap<Cow<'static, str>, StaticValue>>()
                 .into(),
-            Value::EndSentinel => panic!("Cannot process sentinel value"),
+            Value::Sentinel => panic!("Cannot process sentinel value"),
             Value::Bytes(t) => Value::from(t.into_owned()),
             Value::DescVal(Reverse(val)) => Value::DescVal(Reverse(val.to_static().into())),
         }
diff --git a/src/runtime/instance.rs b/src/runtime/instance.rs
index 6e4414ee..8fc6c325 100644
--- a/src/runtime/instance.rs
+++ b/src/runtime/instance.rs
@@ -1,7 +1,7 @@
 use std::{mem, result};
-use std::collections::BTreeMap;
+use std::collections::{BTreeMap, BTreeSet};
 use cozorocks::{BridgeError, DbPtr, destroy_db, OptionsPtrShared, PinnableSlicePtr, ReadOptionsPtr, TDbOptions, TransactionPtr, TransactOptions, WriteOptionsPtr};
-use std::sync::{Arc, LockResult, Mutex, PoisonError};
+use std::sync::{Arc, LockResult, Mutex, PoisonError, RwLock, RwLockReadGuard, RwLockWriteGuard};
 use std::sync::atomic::{AtomicU32, Ordering};
 use lazy_static::lazy_static;
 use log::error;
@@ -10,6 +10,7 @@ use crate::data::tuple::{DataKind, OwnTuple, Tuple, TupleError};
 use crate::data::tuple_set::MIN_TABLE_ID_BOUND;
 use crate::data::typing::Typing;
 use crate::data::value::{StaticValue, Value};
+use crate::runtime::instance::DbInstanceError::TableDoesNotExist;
 use crate::runtime::options::{default_options, default_read_options, default_txn_db_options, default_txn_options, default_write_options};
 
 #[derive(thiserror::Error, Debug)]
@@ -22,6 +23,15 @@ pub enum DbInstanceError {
 
     #[error(transparent)]
     Tuple(#[from] TupleError),
+
+    #[error("Cannot obtain table access lock")]
+    TableAccessLock,
+
+    #[error("Cannot obtain table mutation lock")]
+    TableMutationLock,
+
+    #[error("Table does not exist: {0}")]
+    TableDoesNotExist(u32),
 }
 
 type Result<T> = result::Result<T, DbInstanceError>;
@@ -41,6 +51,8 @@ struct SessionHandle {
     status: SessionStatus,
 }
 
+type TableLock = Arc<RwLock<()>>;
+
 pub struct DbInstance {
     pub(crate) main: DbPtr,
     options: OptionsPtrShared,
@@ -49,6 +61,7 @@ pub struct DbInstance {
     session_handles: Mutex<Vec<Arc<Mutex<SessionHandle>>>>,
     optimistic: bool,
     destroy_on_close: bool,
+    table_locks: TableLock,
 }
 
 impl DbInstance {
@@ -64,6 +77,7 @@ impl DbInstance {
             path: path.to_string(),
             session_handles: vec![].into(),
             destroy_on_close: false,
+            table_locks: Default::default(),
         })
     }
 }
@@ -123,6 +137,7 @@ impl DbInstance {
             stack: vec![],
             cur_table_id: 0.into(),
             params: Default::default(),
+            table_locks: self.table_locks.clone(),
         })
     }
 
@@ -181,7 +196,7 @@ impl Drop for DbInstance {
 enum SessionDefinable {
     Value(StaticValue),
     Expr(StaticExpr),
-    Typing(Typing)
+    Typing(Typing),
     // TODO
 }
 
@@ -199,19 +214,16 @@ pub struct Session {
     stack: Vec<SessionStackFrame>,
     params: BTreeMap<String, StaticValue>,
     session_handle: Arc<Mutex<SessionHandle>>,
+    table_locks: TableLock,
 }
 
 pub(crate) struct InterpretContext<'a> {
     session: &'a Session,
 }
 
-impl <'a> InterpretContext<'a> {
-    pub(crate) fn resolve(&self, key: impl AsRef<str>) {
-
-    }
-    pub(crate) fn resolve_value(&self, key: impl AsRef<str>) {
-
-    }
+impl<'a> InterpretContext<'a> {
+    pub(crate) fn resolve(&self, key: impl AsRef<str>) {}
+    pub(crate) fn resolve_value(&self, key: impl AsRef<str>) {}
     pub(crate) fn resolve_typing(&self, key: impl AsRef<str>) {
         todo!()
     }
@@ -291,6 +303,12 @@ impl Session {
         txn.commit()?;
         Ok(cur_id + 1)
     }
+    pub(crate) fn table_access_guard(&self, ids: BTreeSet<u32>) -> Result<RwLockReadGuard<()>> {
+            self.table_locks.try_read().map_err(|_| DbInstanceError::TableAccessLock)
+    }
+    pub(crate) fn table_mutation_guard(&self, ids: BTreeSet<u32>) -> Result<RwLockWriteGuard<()>> {
+        self.table_locks.write().map_err(|_| DbInstanceError::TableAccessLock)
+    }
 }
 
 lazy_static! {