MonetDB: const_aggr_elim - Adds some comments for clarity

[stefanos mavros via checkin-list]

Changeset: 2b345d7a347d for MonetDB
URL: https://dev.monetdb.org/hg/MonetDB/rev/2b345d7a347d
Modified Files:
        sql/server/rel_optimize_proj.c
Branch: const_aggr_elim
Log Message:

Adds some comments for clarity


diffs (54 lines):

diff --git a/sql/server/rel_optimize_proj.c b/sql/server/rel_optimize_proj.c
--- a/sql/server/rel_optimize_proj.c
+++ b/sql/server/rel_optimize_proj.c
@@ -2515,6 +2515,11 @@ rel_remove_const_aggr(visitor *v, sql_re
        }
 
        if(!list_empty(rel->r)) {
+               /* in the general case in an expression of an aggregate over
+                * a constant can be rewritten as just the const e.g.
+                *   aggr(const) -> const
+                */
+
                for(node *n = exps->h; n; n = n->next) {
                        sql_exp *e = n->data;
 
@@ -2524,11 +2529,17 @@ rel_remove_const_aggr(visitor *v, sql_re
 
                        sql_func *j = ((sql_subfunc *)e->f)->func;
 
+                       /* some aggregates with const values can only be 
eliminated
+                        * under certain circumstances e.g.
+                        *   sum(NULL)   -> NULL, sum(0)  -> 0
+                        *   prod(NULL)  -> NULL, prod(1) -> 1
+                        *   count(NULL) -> 0
+                        */
                        int sum = strcmp(j->base.name, "sum") == 0,
                                prd = strcmp(j->base.name, "prod") == 0,
                                cnt = strcmp(j->base.name, "count") == 0;
 
-                       if(!j->s && j->system == 1) { // && !cnt
+                       if(!j->s && j->system == 1) {
                                list *se = e->l;
 
                                if(se == NULL) {
@@ -2549,7 +2560,6 @@ rel_remove_const_aggr(visitor *v, sql_re
                                                        continue;
                                                }
 
-                                               /* Handle: select 
count(distinct NULL) + 3 == 3 */
                                                if(cnt) {
                                                        if(wa->isnull) {
                                                                
list_remove_node(se, NULL, m);
@@ -2572,9 +2582,9 @@ rel_remove_const_aggr(visitor *v, sql_re
                }
        }
 
-       /* 
+       /*
         * Below code replaces GROUP BY with PROJECT in some cases;
-        * Triggers on... 
+        * Triggers on...
         * select 1 having true; select 42 from foo group by x; select n from 
foo group by rollup(n);
        */
 
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