Add docstrings.

sql/plan/indexed_table_access.go

@@ -27,6 +27,9 @@ var ErrNoIndexableTable = errors.NewKind("expected an IndexableTable, couldn't f
 var ErrNoIndexedTableAccess = errors.NewKind("expected an IndexedTableAccess, couldn't find one in %v")
 var ErrInvalidLookupForIndexedTable = errors.NewKind("indexable table does not support given lookup: %s")
 
+// TableNode is an interface for nodes that are also tables. A node that implements this interface exposes all the
+// information needed for filters on the table to be optimized into indexes. This is possible when the return value
+// of `UnderlyingTable` is a table that implements `sql.IndexAddressable`
 type TableNode interface {
 	sql.Table
 	sql.Node

sql/plan/resolved_table.go

@@ -30,6 +30,7 @@ type ResolvedTable struct {
 	comment     string
 }
 
+// UnderlyingTable returns the table wrapped by the ResolvedTable.
 func (t *ResolvedTable) UnderlyingTable() sql.Table {
 	if w, ok := t.Table.(sql.TableWrapper); ok {
 		return w.Underlying()