[Wasm] Adapt the cost function of Filter.

`Filter` must be more expensive than `LazyDisjunctiveFilter` on
disjunctive predicates.

Author: ImmanuelHaffner

src/backend/WasmOperator.cpp

@@ -458,6 +458,16 @@ ConditionSet Filter<Predicated>::adapt_post_condition(const Match<Filter>&, cons
     return post_cond;
 }
 
+template<bool Predicated>
+double Filter<Predicated>::cost(const Match<Filter> &M)
+{
+    const cnf::CNF &cond = M.filter.filter();
+    const unsigned cost = std::accumulate(cond.cbegin(), cond.cend(), 0U, [](unsigned cost, const cnf::Clause &clause) {
+        return cost + clause.size();
+    });
+    return cost * (Predicated ? 2.0 : 1.0);
+}
+
 template<bool Predicated>
 void Filter<Predicated>::execute(const Match<Filter> &M, setup_t setup, pipeline_t pipeline, teardown_t teardown)
 {
@@ -490,7 +500,7 @@ double LazyDisjunctiveFilter::cost(const Match<LazyDisjunctiveFilter> &M)
 {
     const cnf::CNF &cond = M.filter.filter();
     M_insist(cond.size() == 1, "disjunctive filter condition must be a single clause");
-    return cond[0].size(); // number of predicates in the clause
+    return cond[0].size() / 2.0; // on avg. half the number of predicates in the clause XXX consider selectivities
 }
 
 void LazyDisjunctiveFilter::execute(const Match<LazyDisjunctiveFilter> &M, setup_t setup, pipeline_t pipeline,

src/backend/WasmOperator.hpp

@@ -106,7 +106,7 @@ template<bool Predicated>
 struct Filter : PhysicalOperator<Filter<Predicated>, FilterOperator>
 {
     static void execute(const Match<Filter> &M, setup_t setup, pipeline_t pipeline, teardown_t teardown);
-    static double cost(const Match<Filter>&) { return M_CONSTEXPR_COND(Predicated, 2.0, 1.0); }
+    static double cost(const Match<Filter>&);
     static ConditionSet adapt_post_condition(const Match<Filter> &M, const ConditionSet &post_cond_child);
 };