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Bug #39653: find_shortest_key in sql_select.cc does not

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consider clustered primary keys

Choosing a shortest index for the covering index scan,
the optimizer ignored the fact, that the clustered primary
key read involves whole table data.

The find_shortest_key function has been modified to
take into account that fact that a clustered PK has a
longest key of possible covering indices.
This commit is contained in:
Gleb Shchepa
2010-03-05 23:45:55 +04:00
parent fd7bf5bcf7
commit 897863813e
3 changed files with 65 additions and 2 deletions
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27
sql/sql_select.cc
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@@ -12912,12 +12912,35 @@ static int test_if_order_by_key(ORDER *order, TABLE *table, uint idx,
uint find_shortest_key(TABLE *table, const key_map *usable_keys)
{
uint min_length= (uint) ~0;
uint best= MAX_KEY;
uint usable_clustered_pk= (table->file->primary_key_is_clustered() &&
table->s->primary_key != MAX_KEY &&
usable_keys->is_set(table->s->primary_key)) ?
table->s->primary_key : MAX_KEY;
if (!usable_keys->is_clear_all())
{
uint min_length= (uint) ~0;
for (uint nr=0; nr < table->s->keys ; nr++)
{
/*
As far as
1) clustered primary key entry data set is a set of all record
fields (key fields and not key fields) and
2) secondary index entry data is a union of its key fields and
primary key fields (at least InnoDB and its derivatives don't
duplicate primary key fields there, even if the primary and
the secondary keys have a common subset of key fields),
then secondary index entry data is always a subset of primary key
entry, and the PK is always longer.
Unfortunately, key_info[nr].key_length doesn't show the length
of key/pointer pair but a sum of key field lengths only, thus
we can't estimate index IO volume comparing only this key_length
value of seconday keys and clustered PK.
So, try secondary keys first, and choose PK only if there are no
usable secondary covering keys:
*/
if (nr == usable_clustered_pk)
continue;
if (usable_keys->is_set(nr))
{
if (table->key_info[nr].key_length < min_length)
@@ -12928,7 +12951,7 @@ uint find_shortest_key(TABLE *table, const key_map *usable_keys)
}
}
}
return best;
return best != MAX_KEY ? best : usable_clustered_pk;
}
/**