diff --git a/sql/item_jsonfunc.cc b/sql/item_jsonfunc.cc
index 75e545ad711..7bb4e3bfebb 100644
--- a/sql/item_jsonfunc.cc
+++ b/sql/item_jsonfunc.cc
@@ -308,6 +308,26 @@ int json_path_compare(const json_path_t *a, const json_path_t *b,
 }
 
 
+/*
+  Ensure *js can take 'add' more bytes, growing the buffer geometrically.
+
+  json_nice() produces output that is larger than its input (indentation and
+  newlines), while String::append() enlarges the buffer by only a constant
+  increment once extra_alloc saturates (see realloc_with_extra()).  Without
+  geometric growth the repeated reallocations copy the whole accumulated
+  result over and over, making the reformatting O(output_length^2).  Reserving
+  in doubling steps keeps the whole operation linear.
+*/
+static inline int json_nice_reserve(String *js, size_t add)
+{
+  size_t need= (size_t) js->length() + add;
+  if (need <= js->alloced_length())
+    return 0;
+  size_t dbl= (size_t) js->alloced_length() * 2;
+  return js->realloc(need > dbl ? need : dbl);
+}
+
+
 static int json_nice(json_engine_t *je, String *nice_js,
                      Item_func_json_format::formats mode, int tab_size=4)
 {
@@ -348,6 +368,16 @@ static int json_nice(json_engine_t *je, String *nice_js,
   do
   {
     curr_state= je->state;
+    /*
+      Reserve (geometrically) for every bounded structural append this
+      iteration may emit in any state: comma, indentation (append_tab),
+      colon, quotes and braces.  This covers the JST_*_END cases too, where
+      closing a deep nest appends indentation with no other reserve.  The
+      unbounded key/value text is reserved for separately below.
+    */
+    if (json_nice_reserve(nice_js,
+          comma_len + colon_len + (size_t) (depth + 1) * tab_size + 16))
+      goto error;
     switch (je->state)
     {
     case JST_KEY:
@@ -370,6 +400,8 @@ static int json_nice(json_engine_t *je, String *nice_js,
             append_tab(nice_js, depth, tab_size))
           goto error;
 
+        if (json_nice_reserve(nice_js, (size_t) (key_end - key_start) + 8))
+          goto error;
         nice_js->append('"');
         if (append_simple(nice_js, key_start, key_end - key_start))
           goto error;
@@ -393,6 +425,9 @@ static int json_nice(json_engine_t *je, String *nice_js,
         goto error;
       if (json_value_scalar(je))
       {
+        if (json_nice_reserve(nice_js,
+              (size_t) (je->value_end - je->value_begin) + 8))
+          goto error;
         if (append_simple(nice_js, je->value_begin,
                           je->value_end - je->value_begin))
           goto error;