diff --git a/pyproject.toml b/pyproject.toml
index 2e3eec3..9e869c6 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "sqlonfhir"
-version = "0.0.1"
+version = "0.0.2"
 authors = [
   { name="Stuart Mackle", email="stuart.mackle@sas.com" },
   { name="SAS Healthcare Solutions", email="support@sas.com" }
diff --git a/sqlonfhir/sqlonfhir.py b/sqlonfhir/sqlonfhir.py
index e4377c5..882a139 100644
--- a/sqlonfhir/sqlonfhir.py
+++ b/sqlonfhir/sqlonfhir.py
@@ -1,165 +1,188 @@
 # Copyright © 2025, SAS Institute Inc., Cary, NC, USA. All Rights Reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-from fhirpathpy import evaluate
+from fhirpathpy import compile
 from fhirpathpy.models import models
 
 
-# This is here to change identified.ofType(boolean) to identifiedBoolean.ofType(boolean)
-# This function should be replaced at some point as fhirpathpy and the SQL on FHIR specifications converge
-def escape_path(path):
-    return path.replace("$this", "identity()")
-
-
-def eval_fhirpath(resource, path):
-    path = escape_path(path)
-    user_invocation_table = {
-        "getReferenceKey": {
-            "fn": get_reference_key,
-            "arity": {0: [], 1: ["Identifier"]},
-        },
-        "getResourceKey": {"fn": get_resource_key},
-        "identity": {"fn": identity},
-    }
-    e = evaluate(
-        resource,
-        path,
-        options={"userInvocationTable": user_invocation_table},
-        model=models["r4"],
-    )
-    return e
-
-
-def identity(resource):
-    return resource
-
-
-def union_all(expr, resource, view_definition):
-    result = []
-    for expression in expr["unionAll"]:
-        result += call_fn(expression, resource, view_definition)
-    return result
-
-
-def for_each(expr, resource, view_definition):
-    result = []
-    replaced_path = replace_constants(expr["forEach"], view_definition)
-    selections = eval_fhirpath(resource, replaced_path)
-    for selection in selections:
-        result += select(expr, selection, view_definition)
-    return result
-
-
-def get_all_child_columns(expression):
-    empty_record = {}
-    if "column" in expression:
-        for column in expression["column"]:
-            empty_record[column["name"]] = None
-    if "select" in expression:
-        for selection in expression["select"]:
-            empty_record = empty_record | get_all_child_columns(selection)
-    if "unionAll" in expression:
-        for selection in expression["unionAll"]:
-            empty_record = empty_record | get_all_child_columns(selection)
-    return empty_record
-
-
-def for_each_or_null(expr, resource, view_definition):
-    result = []
-    selections = eval_fhirpath(resource, expr["forEachOrNull"])
-    if len(selections) == 0:
-        return [get_all_child_columns(expr)]
-    for selection in selections:
-        result += select(expr, selection, view_definition)
-    return result
-
-
-def row_product(parts):
-    if len(parts) == 1:
-        return parts[0]
-    rows = [{}]
-    new_rows = None
-    for part in parts:
-        new_rows = []
-        for partial_row in part:
-            for row in rows:
-                new_rows.append(partial_row | row)
-        rows = new_rows
-    return rows
-
-
-def select(expr, resource, view_definition):
-    if "where" in expr:
-        for condition in expr["where"]:
-            replaced_path = replace_constants(condition["path"], view_definition)
-            val = eval_fhirpath(resource, replaced_path)
-            if len(val) == 0:
-                return []
-            elif not val[0]:
-                return []
-            elif type(val[0]) is not bool:
-                raise Exception("Where clause did not evaluate to boolean")
-    sub_selections = []
-    for selection in expr["select"]:
-        selection_evaluation = call_fn(selection, resource, view_definition)
-        if selection_evaluation != []:
-            sub_selections.append(selection_evaluation)
-        else:
-            return []
-    return row_product(sub_selections)
-
-
-def column(expr, resource, view_definition):
-    record = {}
-    for column in expr["column"]:
-        replaced_path = replace_constants(column["path"], view_definition)
-        value = eval_fhirpath(resource, replaced_path)
-        if "collection" in column and column["collection"]:
-            record[column["name"]] = value
-        elif len(value) == 1:
-            record[column["name"]] = value[0]
-        elif len(value) == 0:
-            record[column["name"]] = None
-        else:
-            raise Exception("Unexpected multiple values")
-    return [record]
-
-
-def call_fn(expr, resource, view_definition):
-    if "forEachOrNull" in expr:
-        return for_each_or_null(expr, resource, view_definition)
-    elif "forEach" in expr:
-        return for_each(expr, resource, view_definition)
-    elif "select" in expr:
-        return select(expr, resource, view_definition)
-    elif "unionAll" in expr:
-        return union_all(expr, resource, view_definition)
-    elif "column" in expr:
-        return column(expr, resource, view_definition)
-
-
-def get_resource_key(ctx):
-    return ctx[0]["id"]
-
-
-def get_reference_key(ctx, identifier=None):
-    if identifier and not ctx[0]["reference"].startswith(identifier):
-        return None
-    index = ctx[0]["reference"].find("/")
-    return ctx[0]["reference"][index + 1 :]
-
-
 def eval(resources, view_definition):
     norm = normalize(view_definition)
     result = []
+    evaluator = ViewDefinitionEvaluator()
     for resource in resources:
         if resource["resourceType"] != view_definition["resource"]:
             continue
-        result += call_fn(norm, resource, view_definition)
+        result += evaluator.call_fn(norm, resource, view_definition)
     return result
 
 
+# View Definition Evaluation
+class ViewDefinitionEvaluator:
+    def __init__(self):
+        self.fhirpath_cache = {}
+
+    def eval_fhirpath(self, resource, path):
+        # $this is not in the FHIRPath spec, replacing as per
+        # reference implementation
+        path = path.replace("$this", "identity()")
+        # Add functions to FHIRPath that are needed for SQL on FHIR
+        user_invocation_table = {
+            "getReferenceKey": {
+                "fn": self.get_reference_key,
+                "arity": {0: [], 1: ["Identifier"]},
+            },
+            "getResourceKey": {"fn": self.get_resource_key},
+            "identity": {"fn": self.identity},
+        }
+
+        if path not in self.fhirpath_cache:
+            self.fhirpath_cache[path] = compile(
+                path,
+                model=models["r4"],
+                options={"userInvocationTable": user_invocation_table},
+            )
+
+        return self.fhirpath_cache[path](resource)
+
+    def union_all(self, expr, resource, view_definition):
+        result = []
+        for expression in expr["unionAll"]:
+            result += self.call_fn(expression, resource, view_definition)
+        return result
+
+    def for_each(self, expr, resource, view_definition):
+        result = []
+        replaced_path = self.replace_constants(expr["forEach"], view_definition)
+        selections = self.eval_fhirpath(resource, replaced_path)
+        for selection in selections:
+            result += self.select(expr, selection, view_definition)
+        return result
+
+    def get_all_child_columns(self, expression):
+        empty_record = {}
+        if "column" in expression:
+            for column in expression["column"]:
+                empty_record[column["name"]] = None
+        if "select" in expression:
+            for selection in expression["select"]:
+                empty_record = empty_record | self.get_all_child_columns(selection)
+        if "unionAll" in expression:
+            for selection in expression["unionAll"]:
+                empty_record = empty_record | self.get_all_child_columns(selection)
+        return empty_record
+
+    def for_each_or_null(self, expr, resource, view_definition):
+        result = []
+        selections = self.eval_fhirpath(resource, expr["forEachOrNull"])
+        if len(selections) == 0:
+            return [self.get_all_child_columns(expr)]
+        for selection in selections:
+            result += self.select(expr, selection, view_definition)
+        return result
+
+    def select(self, expr, resource, view_definition):
+        if "where" in expr:
+            for condition in expr["where"]:
+                replaced_path = self.replace_constants(
+                    condition["path"], view_definition
+                )
+                val = self.eval_fhirpath(resource, replaced_path)
+                if len(val) == 0 or not val[0]:
+                    return []
+                elif type(val[0]) is not bool:
+                    raise Exception("Where clause did not evaluate to boolean")
+        sub_selections = []
+        for selection in expr["select"]:
+            selection_evaluation = self.call_fn(selection, resource, view_definition)
+            if selection_evaluation != []:
+                sub_selections.append(selection_evaluation)
+            else:
+                return []
+        return self.row_product(sub_selections)
+
+    def column(self, expr, resource, view_definition):
+        record = {}
+        for column in expr["column"]:
+            replaced_path = self.replace_constants(column["path"], view_definition)
+            value = self.eval_fhirpath(resource, replaced_path)
+            if "collection" in column and column["collection"]:
+                record[column["name"]] = value
+            elif len(value) == 1:
+                record[column["name"]] = value[0]
+            elif len(value) == 0:
+                record[column["name"]] = None
+            else:
+                raise Exception("Unexpected multiple values")
+        return [record]
+
+    def call_fn(self, expr, resource, view_definition):
+        if "forEachOrNull" in expr:
+            return self.for_each_or_null(expr, resource, view_definition)
+        elif "forEach" in expr:
+            return self.for_each(expr, resource, view_definition)
+        elif "select" in expr:
+            return self.select(expr, resource, view_definition)
+        elif "unionAll" in expr:
+            return self.union_all(expr, resource, view_definition)
+        elif "column" in expr:
+            return self.column(expr, resource, view_definition)
+
+    # Utility functions
+    @staticmethod
+    def row_product(parts):
+        if len(parts) == 1:
+            return parts[0]
+        rows = [{}]
+        new_rows = None
+        for part in parts:
+            new_rows = []
+            for partial_row in part:
+                for row in rows:
+                    new_rows.append(partial_row | row)
+            rows = new_rows
+        return rows
+
+    @staticmethod
+    def replace_constants(path, view_definition):
+        for constant in view_definition.get("constant") or []:
+            value_key = [vk for vk in constant.keys() if vk.startswith("value")]
+            if "valueBoolean" in constant:
+                path = path.replace(
+                    "%" + constant["name"], str(constant["valueBoolean"]).lower()
+                )
+            elif value_key[0] in [
+                "valueInteger",
+                "valueDecimal",
+                "valueUnsignedInt",
+                "valuePositiveInt",
+            ]:
+                path = path.replace("%" + constant["name"], str(constant[value_key[0]]))
+            elif len(value_key) == 1:
+                path = path.replace(
+                    "%" + constant["name"], "'" + constant[value_key[0]] + "'"
+                )
+        return path
+
+    # FHIRPath Helper Functions
+    @staticmethod
+    def get_resource_key(ctx):
+        return ctx[0]["id"]
+
+    @staticmethod
+    def get_reference_key(ctx, identifier=None):
+        if identifier and not ctx[0]["reference"].startswith(identifier):
+            return None
+        index = ctx[0]["reference"].find("/")
+        return ctx[0]["reference"][index + 1 :]
+
+    @staticmethod
+    def identity(resource):
+        return resource
+
+
+# View Definition Normalization & Validation
 def normalize(view):
+    # Make sure we only operate on keys we have implemented for
     current_functions = view.keys() & {
         "select",
         "column",
@@ -170,6 +193,7 @@ def normalize(view):
     if "forEach" in view or "forEachOrNull" in view:
         if "select" not in view:
             view["select"] = []
+        # Move column & unionAll under forEach for evaluation for_each() -> union_all()/column()
         view = move_functions(
             view, "select", current_functions - {"forEach", "select", "forEachOrNull"}
         )
@@ -177,7 +201,9 @@ def normalize(view):
     elif "select" in view:
         view = move_functions(view, "select", current_functions - {"select"})
         view["select"] = [normalize(selection) for selection in view["select"]]
-    # if unionAll and column are present, we want to select so they are merged at the same level
+    # if unionAll and column are present make sure it is evaluated as row_product(unionAll + column)
+    # instead of union_all(column()). We also require select() to make sure both get evaluated
+    # and union_all doesn't take precedence
     elif "unionAll" in view and "column" in view:
         view["select"] = []
         view = move_functions(view, "select", current_functions - {"select"})
@@ -199,27 +225,6 @@ def move_functions(view, function, sub_functions):
     return view
 
 
-def replace_constants(path, view_definition):
-    for constant in view_definition.get("constant") or []:
-        value_key = [vk for vk in constant.keys() if vk.startswith("value")]
-        if "valueBoolean" in constant:
-            path = path.replace(
-                "%" + constant["name"], str(constant["valueBoolean"]).lower()
-            )
-        elif value_key[0] in [
-            "valueInteger",
-            "valueDecimal",
-            "valueUnsignedInt",
-            "valuePositiveInt",
-        ]:
-            path = path.replace("%" + constant["name"], str(constant[value_key[0]]))
-        elif len(value_key) == 1:
-            path = path.replace(
-                "%" + constant["name"], "'" + constant[value_key[0]] + "'"
-            )
-    return path
-
-
 def validate_union_all(union_all):
     all_column_orderings = []
     for expression in union_all:
diff --git a/uv.lock b/uv.lock
index 4277075..00a3afe 100644
--- a/uv.lock
+++ b/uv.lock
@@ -148,7 +148,7 @@ wheels = [
 
 [[package]]
 name = "sqlonfhir"
-version = "0.0.1"
+version = "0.0.2"
 source = { editable = "." }
 dependencies = [
     { name = "fhirpathpy" },