Bug #46175: NULL read_view and consistent read assertion

The SE API requires mysql to notify the storage engine that
it's going to read certain tables at the beginning of the 
statement (by calling start_stmt(), store_lock() or
external_lock()).
These are typically called by the lock_tables(). 
However SHOW CREATE TABLE is not pre-locking the tables
because it's not expected to access the data at all.
But for some view definitions (that include comparing a
date/datetime/timestamp column to a string returning
scalar subquery) the JOIN::prepare may still access data
when materializing the scalar non-correlated subquery
in Arg_comparator::can_compare_as_dates().
Fixed by not materializing the subquery when the function
is called in a SHOW/EXPLAIN/CREATE VIEW

1 files changed, 8 insertions, 2 deletions

diff --git a/sql/item_cmpfunc.cc b/sql/item_cmpfunc.cc
index c6b88cd8188..0f02b83ade4 100644
--- a/sql/item_cmpfunc.cc
+++ b/sql/item_cmpfunc.cc
@@ -785,15 +785,21 @@ Arg_comparator::can_compare_as_dates(Item *a, Item *b, ulonglong *const_value)
 
   if (cmp_type != CMP_DATE_DFLT)
   {
+    THD *thd= current_thd;
     /*
       Do not cache GET_USER_VAR() function as its const_item() may return TRUE
       for the current thread but it still may change during the execution.
+      Don't use cache while in the context analysis mode only (i.e. for 
+      EXPLAIN/CREATE VIEW and similar queries). Cache is useless in such 
+      cases and can cause problems. For example evaluating subqueries can 
+      confuse storage engines since in context analysis mode tables 
+      aren't locked.
     */
-    if (cmp_type != CMP_DATE_WITH_DATE && str_arg->const_item() &&
+    if (!thd->is_context_analysis_only() &&
+        cmp_type != CMP_DATE_WITH_DATE && str_arg->const_item() &&
         (str_arg->type() != Item::FUNC_ITEM ||
         ((Item_func*)str_arg)->functype() != Item_func::GUSERVAR_FUNC))
     {
-      THD *thd= current_thd;
       ulonglong value;
       bool error;
       String tmp, *str_val= 0;