MFH:

Memory usage optimisation. mysqlnd is not libmysql. mysqlnd does use the
Zend allocator, which means that is easier to hit memory_limit if you
have big stored (buffered) result sets. Before with libmysql you won't
hit memory_limit because libmysql uses libc's allocator and nothing is
checked. Now, with mysqlnd the situation is stricter and it is easier to
hit memory_limit. We try to optimize for big result sets. If a result set
is larger than 10 rows we will start freeing some data to keep memory usage
after 10 rows constant. This will help in the cases where a buffered result
set is scrolled forward only and just only once, or mysqlnd will need to
decode data from the network buffers again - yes, it is a trade-off between
CPU time and memory size. The best for big result sets is of course using
unbuffered queries - for comparison : 3 Million rows with buffered take
at least 180MB, with buffered you will stay at 3MB, and unbuffered will be
just 7-8% slower.

ext/mysqlnd/mysqlnd_debug.c

@@ -830,6 +830,9 @@ void * _mysqlnd_perealloc(void *ptr, size_t new_size, zend_bool persistent MYSQL
 void _mysqlnd_efree(void *ptr MYSQLND_MEM_D)
 {
 	DBG_ENTER(mysqlnd_efree_name);
+	if (!ptr) {
+		DBG_VOID_RETURN;
+	}
 #ifdef MYSQLND_THREADED
 	if (MYSQLND_G(thread_id) != tsrm_thread_id()) {
 		DBG_RETURN(_mysqlnd_pefree(ptr, 1 TSRMLS_CC ZEND_FILE_LINE_CC ZEND_FILE_LINE_EMPTY_CC));