diff --git a/src/backend/access/gist/gistbuild.c b/src/backend/access/gist/gistbuild.c
index f46a42197c..ec28bfe89f 100644
--- a/src/backend/access/gist/gistbuild.c
+++ b/src/backend/access/gist/gistbuild.c
@@ -461,6 +461,21 @@ gist_indexsortbuild(GISTBuildState *state)
 
 	pfree(pagestate->page);
 	pfree(pagestate);
+
+	/*
+	 * When we WAL-logged index pages, we must nonetheless fsync index files.
+	 * Since we're building outside shared buffers, a CHECKPOINT occurring
+	 * during the build has no way to flush the previously written data to
+	 * disk (indeed it won't know the index even exists).  A crash later on
+	 * would replay WAL from the checkpoint, therefore it wouldn't replay our
+	 * earlier WAL entries. If we do not fsync those pages here, they might
+	 * still not be on disk when the crash occurs.
+	 */
+	if (RelationNeedsWAL(state->indexrel))
+	{
+		RelationOpenSmgr(state->indexrel);
+		smgrimmedsync(state->indexrel->rd_smgr, MAIN_FORKNUM);
+	}
 }
 
 /*