Simplify max_workers_per_postgres_scan

* Setting this variable to `0` disables parallelization
* Cardinality of table less than 65536 use only single parallel process
* Higher cardinality will try to use `max_workers_per_postgres_scan`
  parallel processes with upper limit of `max_parallel_workers`

src/pgduckdb.cpp

@@ -4,6 +4,7 @@ extern "C" {
 #include "postgres.h"
 #include "miscadmin.h"
 #include "utils/guc.h"
+#include "postmaster/bgworker_internals.h"
 }
 
 #include "pgduckdb/pgduckdb.h"
@@ -161,7 +162,7 @@ DuckdbInitGUC(void) {
 
 	DefineCustomVariable("duckdb.max_workers_per_postgres_scan",
 	                     "Maximum number of PostgreSQL workers used for a single Postgres scan",
-	                     &duckdb_max_workers_per_postgres_scan, 2, 8);
+	                     &duckdb_max_workers_per_postgres_scan, 0, MAX_PARALLEL_WORKER_LIMIT);
 
 	DefineCustomVariable("duckdb.postgres_role",
 	                     "Which postgres role should be allowed to use DuckDB execution, use the secrets and create "

src/scan/postgres_table_reader.cpp

@@ -156,12 +156,26 @@ PostgresTableReader::PostgresTableReaderCleanup() {
 	table_scan_query_desc = nullptr;
 }
 
+
+/*
+ * Logic is straightforward, if `duckdb_max_workers_per_postgres_scan` is set to 0 we don't want any
+ * parallelization. For cardinality less equal than 2^16 we only try to run one parallel process. When cardinality
+ * is bigger than we should spawn numer of parallel processes set by `duckdb_max_workers_per_postgres_scan` but
+ * not bigger than `max_parallel_workers`.
+ */
+
 int
 PostgresTableReader::ParallelWorkerNumber(Cardinality cardinality) {
-	static const int base_log = 8;
-	int cardinality_log = std::log2(cardinality);
-	int base = cardinality_log / base_log;
-	return std::max(1, std::min(base, std::max(duckdb_max_workers_per_postgres_scan, max_parallel_workers)));
+	static const int cardinality_threshold = 1 << 16;
+	/* No parallel scan wanted */
+	if (!duckdb_max_workers_per_postgres_scan) {
+		return 0;
+	}
+	/* */
+	if (cardinality <= cardinality_threshold) {
+		return 1;
+	}
+	return std::min(duckdb_max_workers_per_postgres_scan, max_parallel_workers);
 }
 
 const char *