feat: Postgres Citus Data HLL plugin implementation (#601) Thanks to @milanbella !

* pg_approx_hll

* pg_approx_hll

* pg_approx_hll

Co-authored-by: milan.bella@ibm.com <milan.bella@ibm.com>

Fixes #563

Author: milanbella

packages/cubejs-schema-compiler/adapter/PostgresQuery.js

@@ -29,6 +29,18 @@ class PostgresQuery extends BaseQuery {
   timeGroupedColumn(granularity, dimension) {
     return `date_trunc('${GRANULARITY_TO_INTERVAL[granularity]}', ${dimension})`;
   }
+
+  hllInit(sql) {
+    return `hll_add_agg(hll_hash_any(${sql}))`;
+  }
+
+  hllMerge(sql) {
+    return `hll_cardinality(hll_union_agg(${sql}))`;
+  }
+
+  countDistinctApprox(sql) {
+    return `hll_cardinality(hll_add_agg(hll_hash_any(${sql})))`;
+  }
 }
 
 module.exports = PostgresQuery;