docs: consistently mark 0 as valid anchor/reference

proto/substrait/algebra.proto

@@ -388,6 +388,8 @@ message ConsistentPartitionWindowRel {
   // sorts, and bounds, since those must be consistent across the various functions in this rel.  Refer
   // to the `WindowFunction` message for a description of these fields.
   message WindowRelFunction {
+    // References a function_anchor defined in this plan.
+    // 0 is a valid anchor/reference.
     uint32 function_reference = 1;
 
     repeated FunctionArgument arguments = 9;
@@ -788,7 +790,8 @@ message ComparisonJoinKey {
       // A custom comparison behavior is used.  This can happen, for example, when using
       // collations, where we might want to do something like a case-insensitive comparison.
       //
-      // This must be a binary function with a boolean return type
+      // This must be a binary function with a boolean return type.
+      // 0 is a valid anchor/reference.
       uint32 custom_function_reference = 2;
     }
   }
@@ -1046,6 +1049,7 @@ message Expression {
     // optionally points to a type_variation_anchor defined in this plan.
     // Applies to all members of union other than the Typed null (which should
     // directly declare the type variation).
+    // 0 is a valid anchor/reference.
     uint32 type_variation_reference = 51;
 
     message VarChar {
@@ -1125,9 +1129,11 @@ message Expression {
     message UserDefined {
       oneof type_anchor_type {
         // points to a type_anchor defined in this plan
+        // 0 is a valid anchor/reference.
         uint32 type_reference = 1;
 
         // points to a type_alias_anchor defined in this plan.
+        // 0 is a valid anchor/reference.
         uint32 type_alias_reference = 5;
       }
 
@@ -1152,6 +1158,7 @@ message Expression {
 
     // Optionally points to a type_variation_anchor defined in this plan for
     // the returned nested type.
+    // 0 is a valid anchor/reference.
     uint32 type_variation_reference = 2;
 
     oneof nested_type {
@@ -1189,8 +1196,8 @@ message Expression {
   // A scalar function call.
   message ScalarFunction {
     // Points to a function_anchor defined in this plan, which must refer
-    // to a scalar function in the associated YAML file. Required; avoid
-    // using anchor/reference zero.
+    // to a scalar function in the associated YAML file.
+    // Required; 0 is a valid anchor/reference.
     uint32 function_reference = 1;
 
     // The arguments to be bound to the function. This must have exactly the
@@ -1649,6 +1656,7 @@ message DynamicParameter {
   Type type = 1;
 
   // The surrogate key used within a plan to reference a specific parameter binding.
+  // 0 is a valid anchor/reference.
   uint32 parameter_reference = 2;
 }
 
@@ -1658,6 +1666,8 @@ message SortField {
 
   oneof sort_kind {
     SortDirection direction = 2;
+    // References a function_anchor defined in this plan.
+    // 0 is a valid anchor/reference.
     uint32 comparison_function_reference = 3;
   }
   enum SortDirection {
@@ -1703,8 +1713,8 @@ enum AggregationPhase {
 // An aggregate function.
 message AggregateFunction {
   // Points to a function_anchor defined in this plan, which must refer
-  // to an aggregate function in the associated YAML file. Required; 0 is
-  // considered to be a valid anchor/reference.
+  // to an aggregate function in the associated YAML file.
+  // Required; 0 is a valid anchor/reference.
   uint32 function_reference = 1;
 
   // The arguments to be bound to the function. This must have exactly the

proto/substrait/extensions/extensions.proto

@@ -15,6 +15,7 @@ message SimpleExtensionURI {
   option deprecated = true;
   // A surrogate key used in the context of a single plan used to reference the
   // URI associated with an extension.
+  // 0 is a valid anchor/reference.
   uint32 extension_uri_anchor = 1;
 
   // The URI where this extension YAML can be retrieved. This is the "namespace"
@@ -25,6 +26,7 @@ message SimpleExtensionURI {
 message SimpleExtensionURN {
   // A surrogate key used in the context of a single plan used to reference the
   // URN associated with an extension.
+  // 0 is a valid anchor/reference.
   uint32 extension_urn_anchor = 1;
 
   // The extension URN that uniquely identifies this extension. This must follow the
@@ -45,15 +47,18 @@ message SimpleExtensionDeclaration {
   message ExtensionType {
     // references the extension_uri_anchor defined for a specific extension URI.
     // this is now deprecated and extension_urn_reference should be used instead.
+    // 0 is a valid anchor/reference.
     uint32 extension_uri_reference = 1 [deprecated = true];
 
     // references the extension_urn_anchor defined for a specific extension URN.
     // If both extension_urn_reference and extension_uri_reference are present,
     // extension_urn_reference takes precedence.
+    // 0 is a valid anchor/reference.
     uint32 extension_urn_reference = 4;
 
     // A surrogate key used in the context of a single plan to reference a
-    // specific extension type
+    // specific extension type.
+    // 0 is a valid anchor/reference.
     uint32 type_anchor = 2;
 
     // the name of the type in the defined extension YAML.
@@ -63,15 +68,18 @@ message SimpleExtensionDeclaration {
   message ExtensionTypeVariation {
     // references the extension_uri_anchor defined for a specific extension URI.
     // this is now deprecated and extension_urn_reference should be used instead.
+    // 0 is a valid anchor/reference.
     uint32 extension_uri_reference = 1 [deprecated = true];
 
     // references the extension_urn_anchor defined for a specific extension URN.
     // If both extension_urn_reference and extension_uri_reference are present,
     // extension_urn_reference takes precedence.
+    // 0 is a valid anchor/reference.
     uint32 extension_urn_reference = 4;
 
     // A surrogate key used in the context of a single plan to reference a
-    // specific type variation
+    // specific type variation.
+    // 0 is a valid anchor/reference.
     uint32 type_variation_anchor = 2;
 
     // the name of the type in the defined extension YAML.
@@ -81,15 +89,18 @@ message SimpleExtensionDeclaration {
   message ExtensionFunction {
     // references the extension_uri_anchor defined for a specific extension URI.
     // this is now deprecated and extension_urn_reference should be used instead.
+    // 0 is a valid anchor/reference.
     uint32 extension_uri_reference = 1 [deprecated = true];
 
     // references the extension_urn_anchor defined for a specific extension URN.
     // If both extension_urn_reference and extension_uri_reference are present,
     // extension_urn_reference takes precedence.
+    // 0 is a valid anchor/reference.
     uint32 extension_urn_reference = 4;
 
     // A surrogate key used in the context of a single plan to reference a
-    // specific function
+    // specific function.
+    // 0 is a valid anchor/reference.
     uint32 function_anchor = 2;
 
     // A function signature

proto/substrait/plan.proto

@@ -102,6 +102,7 @@ message Version {
 // Represents a binding for a dynamic parameter.
 message DynamicParameterBinding {
   // The parameter anchor that identifies the dynamic parameter reference.
+  // 0 is a valid anchor/reference.
   uint32 parameter_anchor = 1;
 
   // The literal value assigned to the parameter at runtime.

proto/substrait/type.proto

@@ -10,6 +10,9 @@ option go_package = "github.com/substrait-io/substrait-protobuf/go/substraitpb";
 option java_multiple_files = true;
 option java_package = "io.substrait.proto";
 
+// Note: type_variation_reference fields within Type messages reference a
+// type_variation_anchor defined in the plan's extension declarations. The value
+// 0 represents the system-preferred variation and is a valid reference value.
 message Type {
   oneof kind {
     Boolean bool = 1;
@@ -224,6 +227,8 @@ message Type {
   }
 
   message UserDefined {
+    // References a type_anchor defined in the plan's extension declarations.
+    // 0 is a valid anchor/reference.
     uint32 type_reference = 1;
     uint32 type_variation_reference = 2;
     Nullability nullability = 3;
@@ -257,6 +262,7 @@ message Type {
 message TypeAlias {
   // A surrogate key used in the context of a single plan to reference a
   // specific type alias.
+  // 0 is a valid anchor/reference.
   uint32 type_alias_anchor = 1;
 
   // A concrete type to be aliased.

site/docs/extensions/index.md

@@ -22,7 +22,7 @@ This extension URN uses the format `extension:<OWNER>:<ID>`, where:
 
 The YAML file is constructed according to the [YAML Schema](https://github.com/substrait-io/substrait/blob/main/text/simple_extensions_schema.yaml). Each definition in the file corresponds to the YAML-based serialization of the relevant data structure. If a user only wants to extend one of these types of objects (e.g. types), a developer does not have to provide definitions for the other extension points.
 
-A Substrait plan can reference one or more YAML files via their extension URN. In the places where these entities are referenced, they will be referenced using an extension URN + name reference. The name scheme per type works as follows:
+A Substrait plan can reference one or more YAML files via their extension URN. In the places where these entities are referenced, they will be referenced using an extension URN + name reference. Each extension entity (type, type variation, or function) is assigned an anchor value, which is a non-negative integer starting from 0. The anchor value 0 is valid and can be used to reference extension entities. The name scheme per type works as follows:
 
 | Category           | Naming scheme                                                |
 | ------------------ | ------------------------------------------------------------ |

site/docs/serialization/binary_serialization.md

@@ -36,6 +36,8 @@ Extension URNs and declarations are encapsulated in the top level of the plan. E
 
 Once the YAML file extension URN anchor is defined, the anchor will be referenced by zero or more `SimpleExtensionDefinition`s. For each simple extension definition, an anchor is defined for that specific extension entity. This anchor is then referenced to within lower-level primitives (functions, etc.) to reference that specific extension. Message properties are named `*_anchor` where the anchor is defined and `*_reference` when referencing the anchor. For example `function_anchor` and `function_reference`.
 
+Anchor values are non-negative integers starting from 0. A value of 0 is valid and can be used to reference an extension entity.
+
 === "Simple Extension Declaration"
 
     ```proto

site/docs/types/type_aliases.md

@@ -10,6 +10,7 @@ Type aliases allow a plan to declare a type once and reference it multiple times
 
 A type alias is a mapping from an anchor to a concrete Substrait type. A valid type alias is described below.
 
+* Anchors are non-negative integers starting from 0, meaning 0 is a valid anchor value.
 * All type parameters must be specified.
 * Cannot directly be another alias.
 * Type parameters can reference other aliased types as long as no circular dependencies are introduced.

site/docs/types/type_classes.md

@@ -50,6 +50,8 @@ Compound type classes are type classes that need to be configured by means of a
 
 User-defined type classes are defined as part of [simple extensions](../extensions/index.md#simple-extensions). An extension can declare an arbitrary number of user-defined extension types. Once a type has been declared, it can be used in function declarations.
 
+User-defined types are referenced in a plan using a `type_reference` anchor value that corresponds to a `type_anchor` defined in the plan's extension declarations. The anchor value is a non-negative integer starting from 0, meaning 0 is a valid anchor value.
+
 For example, the following declares a type named `point` (namespaced to the associated YAML file) and two scalar functions that operate on it.
 
 ```yaml