/*
Constrain the WHERE condition to reflect the aggregate query. For
example:
SELECT AVG(v) FROM widget w;
is rewritten as:
SELECT AVG(v) FROM widget w WHERE v IS NOT NULL;
This enables the query to use an index on v.
*/
func constrainAggregate(cond expression.Expression, aggs map[string]algebra.Aggregate) expression.Expression {
var first expression.Expression
for _, agg := range aggs {
if first == nil {
first = agg.Operand()
if first == nil {
return cond
}
continue
}
op := agg.Operand()
if op == nil || !first.EquivalentTo(op) {
return cond
}
}
if first == nil {
return cond
}
var constraint expression.Expression = expression.NewIsNotNull(first)
if cond != nil {
constraint = expression.NewAnd(cond, constraint)
}
return constraint
}
func (this *builder) buildJoinScan(keyspace datastore.Keyspace, node *algebra.KeyspaceTerm, op string) (
datastore.Index, expression.Covers, error) {
indexes, err := allIndexes(keyspace)
if err != nil {
return nil, nil, err
}
var pred expression.Expression
pred = expression.NewIsNotNull(node.Keys().Copy())
dnf := NewDNF()
pred, err = dnf.Map(pred)
if err != nil {
return nil, nil, err
}
subset := pred
if this.where != nil {
subset = expression.NewAnd(subset, this.where.Copy())
subset, err = dnf.Map(subset)
if err != nil {
return nil, nil, err
}
}
formalizer := expression.NewFormalizer()
formalizer.Keyspace = node.Alias()
primaryKey := expression.Expressions{
expression.NewField(
expression.NewMeta(expression.NewConstant(node.Alias())),
expression.NewFieldName("id", false)),
}
sargables, err := sargableIndexes(indexes, pred, subset, primaryKey, dnf, formalizer)
if err != nil {
return nil, nil, err
}
minimals, err := minimalIndexes(sargables, pred)
if err != nil {
return nil, nil, err
}
if len(minimals) == 0 {
return nil, nil, errors.NewNoIndexJoinError(node.Alias(), op)
}
return this.buildCoveringJoinScan(minimals, node, op)
}