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MCOL-4543 Subquery optimization.

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For a query of the form:

SELECT COUNT(c2) FROM (SELECT * FROM t1) q;

where t1 contains 10 columns c1, c2, ... , c10.

We currently create an intermediate RowGroup in ExeMgr with
a row of the form (1, c2_value1, 1, 1, 1, 1, 1, 1, 1, 1), i.e.
for all the columns of the subquery which are not referenced in
the outer query, we substitute a constant value, which is wasteful.

With this optimization, we are trimming the RowGroup to a row
of the form (1, c2_value1). This can have non-trivial query
execution time improvements if the subquery contains large number
of columns (such as a "select *" on a very wide table) and the outer
query is only referencing a subset of these columns with lower
index values from the subquery (as an example, c1 or c2 above).
That is, the current limitation of this optimization is we are not
removing those non-referenced subquery columns (c1 in the query above)
which are to the left of a referenced column.
This commit is contained in:
Gagan Goel
2021-02-26 07:36:29 -05:00
parent 2eec956977
commit 8a03e6c7d1
5 changed files with 359 additions and 8 deletions
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39
dbcon/mysql/ha_mcs_execplan.cpp
View File

@ -2920,15 +2920,46 @@ SimpleColumn* getSmallestColumn(boost::shared_ptr<CalpontSystemCatalog> csc,
if (tan.alias == csep->derivedTbAlias())
{
assert (!csep->returnedCols().empty());
ReturnedColumn* rc = dynamic_cast<ReturnedColumn*>(csep->returnedCols()[0].get());
const CalpontSelectExecutionPlan::ReturnedColumnList& cols = csep->returnedCols();
CalpontSelectExecutionPlan::ReturnedColumnList::const_iterator iter;
ReturnedColumn* rc;
for (iter = cols.begin(); iter != cols.end(); iter++)
{
if ((*iter)->refCount() != 0)
{
rc = dynamic_cast<ReturnedColumn*>(iter->get());
break;
}
}
if (iter == cols.end())
{
assert (!cols.empty());
// We take cols[0] here due to the optimization happening in
// derivedTableOptimization. All cols with refCount 0 from
// the end of the cols list are optimized out, until the
// first column with non-zero refCount is encountered. So
// here, if instead of cols[0], we take cols[1] (based on
// some logic) and increment it's refCount, then cols[0] is
// not optimized out in derivedTableOptimization and is
// added as a ConstantColumn to the derived table's returned
// column list. This later causes an ineffective row group
// with row of the form (1, cols[1]_value1) to be created in ExeMgr.
rc = dynamic_cast<ReturnedColumn*>(cols[0].get());
// @bug5634 derived table optimization.
rc->incRefCount();
}
SimpleColumn* sc = new SimpleColumn();
sc->columnName(rc->alias());
sc->sequence(0);
sc->tableAlias(tan.alias);
sc->timeZone(gwi.thd->variables.time_zone->get_name()->ptr());
// @bug5634 derived table optimization.
rc->incRefCount();
sc->derivedTable(csep->derivedTbAlias());
sc->derivedRefCol(rc);
return sc;