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Bug#52357: Assertion failed: join->best_read in

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The algorithm inside restore_prev_nj_state failed to
properly update the counters within the NESTED_JOIN
tree. The counter was decremented each time a table in the
node was removed from the QEP, the correct thing to do being
only to decrement it when the last table in the child node
was removed from the plan. This lead to node counters
getting negative values and the plan thus appeared
impossible. An assertion caught this.

Fixed by not recursing up the tree unless the last table in
the join nest node is removed from the plan
This commit is contained in:
Martin Hansson
2010-05-06 10:45:00 +02:00
parent 6ae9c70109
commit 4e75f7c053
4 changed files with 113 additions and 12 deletions
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56
sql/sql_select.cc
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@@ -8706,6 +8706,39 @@ static bool check_interleaving_with_nj(JOIN_TAB *next_tab)
/*
Nested joins perspective: Remove the last table from the join order
The algorithm is the reciprocal of check_interleaving_with_nj(), hence
parent join nest nodes are updated only when the last table in its child
node is removed. The ASCII graphic below will clarify.
%A table nesting such as <tt> t1 x [ ( t2 x t3 ) x ( t4 x t5 ) ] </tt>is
represented by the below join nest tree.
@verbatim
NJ1
_/ / \
_/ / NJ2
_/ / / \
/ / / \
t1 x [ (t2 x t3) x (t4 x t5) ]
@endverbatim
At the point in time when check_interleaving_with_nj() adds the table t5 to
the query execution plan, QEP, it also directs the node named NJ2 to mark
the table as covered. NJ2 does so by incrementing its @c counter
member. Since all of NJ2's tables are now covered by the QEP, the algorithm
proceeds up the tree to NJ1, incrementing its counter as well. All join
nests are now completely covered by the QEP.
restore_prev_nj_state() does the above in reverse. As seen above, the node
NJ1 contains the nodes t2, t3, and NJ2. Its counter being equal to 3 means
that the plan covers t2, t3, and NJ2, @e and that the sub-plan (t4 x t5)
completely covers NJ2. The removal of t5 from the partial plan will first
decrement NJ2's counter to 1. It will then detect that NJ2 went from being
completely to partially covered, and hence the algorithm must continue
upwards to NJ1 and decrement its counter to 2. %A subsequent removal of t4
will however not influence NJ1 since it did not un-cover the last table in
NJ2.
SYNOPSIS
restore_prev_nj_state()
last join table to remove, it is assumed to be the last in current
@@ -8722,19 +8755,20 @@ static void restore_prev_nj_state(JOIN_TAB *last)
{
TABLE_LIST *last_emb= last->table->pos_in_table_list->embedding;
JOIN *join= last->join;
while (last_emb)
for (;last_emb != NULL; last_emb= last_emb->embedding)
{
if (!(--last_emb->nested_join->counter))
join->cur_embedding_map&= ~last_emb->nested_join->nj_map;
else if (last_emb->nested_join->join_list.elements-1 ==
last_emb->nested_join->counter)
{
join->cur_embedding_map|= last_emb->nested_join->nj_map;
NESTED_JOIN *nest= last_emb->nested_join;
DBUG_ASSERT(nest->counter > 0);
bool was_fully_covered= nest->is_fully_covered();
if (--nest->counter == 0)
join->cur_embedding_map&= ~nest->nj_map;
if (!was_fully_covered)
break;
}
else
break;
last_emb= last_emb->embedding;
join->cur_embedding_map|= nest->nj_map;
}
}