refactor(mito2): reduce duplicate IndexOutput struct (#4592)

* refactor(mito2): reduce duplicate IndexOutput struct

* docs(mito2): add index output note

src/mito2/src/sst/index.rs

@@ -52,39 +52,27 @@ pub struct IndexOutput {
     pub fulltext_index: FulltextIndexOutput,
 }
 
+/// Base output of the index creation.
+#[derive(Debug, Clone, Default)]
+pub struct IndexBaseOutput {
+    /// Size of the index.
+    pub index_size: ByteCount,
+    /// Number of rows in the index.
+    pub row_count: RowCount,
+    /// Available columns in the index.
+    pub columns: Vec<ColumnId>,
+}
+
+impl IndexBaseOutput {
+    pub fn is_available(&self) -> bool {
+        self.index_size > 0
+    }
+}
+
 /// Output of the inverted index creation.
-#[derive(Debug, Clone, Default)]
-pub struct InvertedIndexOutput {
-    /// Size of the index.
-    pub index_size: ByteCount,
-    /// Number of rows in the index.
-    pub row_count: RowCount,
-    /// Available columns in the index.
-    pub columns: Vec<ColumnId>,
-}
-
+pub type InvertedIndexOutput = IndexBaseOutput;
 /// Output of the fulltext index creation.
-#[derive(Debug, Clone, Default)]
-pub struct FulltextIndexOutput {
-    /// Size of the index.
-    pub index_size: ByteCount,
-    /// Number of rows in the index.
-    pub row_count: RowCount,
-    /// Available columns in the index.
-    pub columns: Vec<ColumnId>,
-}
-
-impl InvertedIndexOutput {
-    pub fn is_available(&self) -> bool {
-        self.index_size > 0
-    }
-}
-
-impl FulltextIndexOutput {
-    pub fn is_available(&self) -> bool {
-        self.index_size > 0
-    }
-}
+pub type FulltextIndexOutput = IndexBaseOutput;
 
 /// The index creator that hides the error handling details.
 #[derive(Default)]