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Fixed bug in the optimiser for FULL TABLE SCAN case:

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to estimate correctly cost of full table scan we should take into
account rows read and skipped on each iteration.
This commit is contained in:
kostja@oak.local
2003-09-15 22:21:39 +04:00
parent 4871a35e8a
commit ab576328f0
4 changed files with 27 additions and 29 deletions
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46
sql/sql_select.cc
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@ -2133,33 +2133,31 @@ find_best(JOIN *join,table_map rest_tables,uint idx,double record_count,
s->table->used_keys && best_key) &&
!(s->table->force_index && best_key))
{ // Check full join
ha_rows rnd_records= s->found_records;
if (s->on_expr)
{
tmp=rows2double(rnd_records); // Can't use read cache
}
else
{
tmp=(double) s->read_time;
/* Calculate time to read previous rows through cache */
tmp*=(1.0+floor((double) cache_record_length(join,idx)*
record_count /
(double) thd->variables.join_buff_size));
}
/*
If there is a restriction on the table, assume that 25% of the
rows can be skipped on next part.
This is to force tables that this table depends on before this
table
*/
if (found_constrain)
rnd_records-= rnd_records/4;
/*
Estimate cost of reading table. Note, that we don't read a table
on each iteration as in most cases join buffer is in use.
*/
tmp= (double) s->read_time;
/*
In case of full scan we check every row in the table:
here we take into account rows read and skipped, as well as rows
passed to next select
*/
if (best == DBL_MAX ||
(tmp + record_count/(double) TIME_FOR_COMPARE*rnd_records <
(tmp + record_count/(double) TIME_FOR_COMPARE*s->records <
best + record_count/(double) TIME_FOR_COMPARE*records))
{
/*
If there is a restriction on the table, assume that 25% of the
rows can be skipped on next part.
This is to force tables that this table depends on before this
table
*/
ha_rows rnd_records= s->found_records;
if (found_constrain)
rnd_records-= rnd_records/4;
/*
If the table has a range (s->quick is set) make_join_select()
will ensure that this will be used