Index: clangd/index/FileIndex.cpp
===================================================================
--- clangd/index/FileIndex.cpp
+++ clangd/index/FileIndex.cpp
@@ -45,9 +45,13 @@
   if (IsIndexMainAST) {
     // We only collect refs when indexing main AST.
     CollectorOpts.RefFilter = RefKind::All;
+    // comments for main file can always be obtained from sema, do not store
+    // them in the index.
+    CollectorOpts.StoreAllDocumentation = false;
   } else {
     IndexOpts.IndexMacrosInPreprocessor = true;
     CollectorOpts.CollectMacro = true;
+    CollectorOpts.StoreAllDocumentation = true;
   }
 
   SymbolCollector Collector(std::move(CollectorOpts));
Index: clangd/index/IndexAction.cpp
===================================================================
--- clangd/index/IndexAction.cpp
+++ clangd/index/IndexAction.cpp
@@ -175,6 +175,7 @@
   Opts.CollectIncludePath = true;
   Opts.CountReferences = true;
   Opts.Origin = SymbolOrigin::Static;
+  Opts.StoreAllDocumentation = false;
   if (RefsCallback != nullptr) {
     Opts.RefFilter = RefKind::All;
     Opts.RefsInHeaders = true;
Index: clangd/index/SymbolCollector.h
===================================================================
--- clangd/index/SymbolCollector.h
+++ clangd/index/SymbolCollector.h
@@ -75,6 +75,10 @@
     /// Collect symbols local to main-files, such as static functions
     /// and symbols inside an anonymous namespace.
     bool CollectMainFileSymbols = true;
+    /// If set to true, SymbolCollector will collect doc for all symbols.
+    /// Note that documents of symbols being indexed for completion will always
+    /// be collected regardless of this option.
+    bool StoreAllDocumentation = false;
     /// If this is set, only collect symbols/references from a file if
     /// `FileFilter(SM, FID)` is true. If not set, all files are indexed.
     std::function<bool(const SourceManager &, FileID)> FileFilter = nullptr;
Index: clangd/index/SymbolCollector.cpp
===================================================================
--- clangd/index/SymbolCollector.cpp
+++ clangd/index/SymbolCollector.cpp
@@ -566,18 +566,13 @@
   std::string Documentation =
       formatDocumentation(*CCS, getDocComment(Ctx, SymbolCompletion,
                                               /*CommentsFromHeaders=*/true));
-  // For symbols not indexed for completion (class members), we also store their
-  // docs in the index, because Sema doesn't load the docs from the preamble, we
-  // rely on the index to get the docs.
-  // FIXME: this can be optimized by only storing the docs in dynamic index --
-  // dynamic index should index these symbols when Sema completes a member
-  // completion.
-  S.Documentation = Documentation;
   if (!(S.Flags & Symbol::IndexedForCodeCompletion)) {
+    if (Opts.StoreAllDocumentation)
+      S.Documentation = Documentation;
     Symbols.insert(S);
     return Symbols.find(S.ID);
   }
-
+  S.Documentation = Documentation;
   std::string Signature;
   std::string SnippetSuffix;
   getSignature(*CCS, &Signature, &SnippetSuffix);
Index: unittests/clangd/SymbolCollectorTests.cpp
===================================================================
--- unittests/clangd/SymbolCollectorTests.cpp
+++ unittests/clangd/SymbolCollectorTests.cpp
@@ -212,6 +212,12 @@
         return WrapperFrontendAction::CreateASTConsumer(CI, InFile);
       }
 
+      bool BeginInvocation(CompilerInstance &CI) override {
+        // Make the compiler parse all comments.
+        CI.getLangOpts().CommentOpts.ParseAllComments = true;
+        return WrapperFrontendAction::BeginInvocation(CI);
+      }
+
     private:
       index::IndexingOptions IndexOpts;
       CommentHandler *PragmaHandler;
@@ -710,6 +716,31 @@
                                    QName("main_f")));
 }
 
+TEST_F(SymbolCollectorTest, Documentation) {
+  const std::string Header = R"(
+    // Doc Foo
+    class Foo {
+      // Doc f
+      int f();
+    };
+  )";
+  CollectorOpts.StoreAllDocumentation = false;
+  runSymbolCollector(Header, /* Main */ "");
+  EXPECT_THAT(Symbols,
+              UnorderedElementsAre(
+                  AllOf(QName("Foo"), Doc("Doc Foo"), ForCodeCompletion(true)),
+                  AllOf(QName("Foo::f"), Doc(""), ReturnType(""),
+                        ForCodeCompletion(false))));
+
+  CollectorOpts.StoreAllDocumentation = true;
+  runSymbolCollector(Header, /* Main */ "");
+  EXPECT_THAT(Symbols,
+              UnorderedElementsAre(
+                  AllOf(QName("Foo"), Doc("Doc Foo"), ForCodeCompletion(true)),
+                  AllOf(QName("Foo::f"), Doc("Doc f"), ReturnType(""),
+                        ForCodeCompletion(false))));
+}
+
 TEST_F(SymbolCollectorTest, ClassMembers) {
   const std::string Header = R"(
     class Foo {