Fix #233 Add limit pushdown to attached queries

src/include/postgres_scanner.hpp

@@ -26,6 +26,7 @@ struct PostgresBindData : public FunctionData {
 	string schema_name;
 	string table_name;
 	string sql;
+	string limit;
 	idx_t pages_approx = 0;
 
 	vector<PostgresType> postgres_types;

src/postgres_scanner.cpp

@@ -251,14 +251,15 @@ static void PostgresInitInternal(ClientContext &context, const PostgresBindData
 	}
 	if (bind_data->table_name.empty()) {
 		D_ASSERT(!bind_data->sql.empty());
-		lstate.sql =
-		    StringUtil::Format(R"(COPY (SELECT %s FROM (%s) AS __unnamed_subquery %s) TO STDOUT (FORMAT "binary");)",
-		                       col_names, bind_data->sql, filter);
+		lstate.sql = StringUtil::Format(
+		    R"(COPY (SELECT %s FROM (%s) AS __unnamed_subquery %s%s) TO STDOUT (FORMAT "binary");)",
+		    col_names, bind_data->sql, filter, bind_data->limit);
 
 	} else {
-		lstate.sql = StringUtil::Format(R"(COPY (SELECT %s FROM %s.%s %s) TO STDOUT (FORMAT "binary");)", col_names,
-		                                KeywordHelper::WriteQuoted(bind_data->schema_name, '"'),
-		                                KeywordHelper::WriteQuoted(bind_data->table_name, '"'), filter);
+		lstate.sql = StringUtil::Format(
+		    R"(COPY (SELECT %s FROM %s.%s %s%s) TO STDOUT (FORMAT "binary");)",
+		    col_names, KeywordHelper::WriteQuoted(bind_data->schema_name, '"'),
+		    KeywordHelper::WriteQuoted(bind_data->table_name, '"'), filter, bind_data->limit);
 	}
 	lstate.exec = false;
 	lstate.done = false;

src/storage/postgres_optimizer.cpp

@@ -3,15 +3,79 @@
 #include "storage/postgres_transaction.hpp"
 #include "storage/postgres_optimizer.hpp"
 #include "duckdb/planner/operator/logical_get.hpp"
+#include "duckdb/planner/operator/logical_limit.hpp"
 #include "storage/postgres_catalog.hpp"
 #include "postgres_scanner.hpp"
 
+
 namespace duckdb {
 
 struct PostgresOperators {
 	reference_map_t<PostgresCatalog, vector<reference<LogicalGet>>> scans;
 };
 
+static void OptimizePostgresScanLimitPushdown(unique_ptr<LogicalOperator> &op) {
+	if (op->type == LogicalOperatorType::LOGICAL_LIMIT) {
+		auto &limit = op->Cast<LogicalLimit>();
+		reference<LogicalOperator> child = *op->children[0];
+
+		while (child.get().type == LogicalOperatorType::LOGICAL_PROJECTION) {
+			child = *child.get().children[0];
+		}
+
+		if (child.get().type != LogicalOperatorType::LOGICAL_GET) {
+			OptimizePostgresScanLimitPushdown(op->children[0]);
+			return;
+		}
+
+		auto &get = child.get().Cast<LogicalGet>();
+		if (!PostgresCatalog::IsPostgresScan(get.function.name)) {
+			OptimizePostgresScanLimitPushdown(op->children[0]);
+			return;
+		}
+
+		switch (limit.limit_val.Type()) {
+		case LimitNodeType::CONSTANT_VALUE:
+		case LimitNodeType::UNSET:
+			break;
+		default:
+			// not a constant or unset limit
+			OptimizePostgresScanLimitPushdown(op->children[0]);
+			return;
+		}
+		switch (limit.offset_val.Type()) {
+		case LimitNodeType::CONSTANT_VALUE:
+		case LimitNodeType::UNSET:
+			break;
+		default:
+			// not a constant or unset offset
+			OptimizePostgresScanLimitPushdown(op->children[0]);
+			return;
+		}
+
+		auto &bind_data = get.bind_data->Cast<PostgresBindData>();
+
+		string generated_limit_clause = "";
+		if (limit.limit_val.Type() != LimitNodeType::UNSET) {
+			generated_limit_clause += " LIMIT " + to_string(limit.limit_val.GetConstantValue());
+		}
+		if (limit.offset_val.Type() != LimitNodeType::UNSET) {
+			generated_limit_clause += " OFFSET " + to_string(limit.offset_val.GetConstantValue());
+		}
+
+		if (!generated_limit_clause.empty()) {
+			bind_data.limit = generated_limit_clause;
+
+			op = std::move(op->children[0]);
+			return;
+		}
+	}
+
+	for (auto &child : op->children) {
+		OptimizePostgresScanLimitPushdown(child);
+	}
+}
+
 void GatherPostgresScans(LogicalOperator &op, PostgresOperators &result) {
 	if (op.type == LogicalOperatorType::LOGICAL_GET) {
 		auto &get = op.Cast<LogicalGet>();
@@ -35,6 +99,8 @@ void GatherPostgresScans(LogicalOperator &op, PostgresOperators &result) {
 }
 
 void PostgresOptimizer::Optimize(OptimizerExtensionInput &input, unique_ptr<LogicalOperator> &plan) {
+	// look at query plan and check if we can find LIMIT/OFFSET to pushdown
+	OptimizePostgresScanLimitPushdown(plan);
 	// look at the query plan and check if we can enable streaming query scans
 	PostgresOperators operators;
 	GatherPostgresScans(*plan, operators);

test/sql/storage/limit.test

@@ -0,0 +1,41 @@
+# name: test/sql/storage/limit.test
+# description: Test limit on an attached table
+# group: [storage]
+
+require postgres_scanner
+
+require-env POSTGRES_TEST_DATABASE_AVAILABLE
+
+statement ok
+ATTACH 'dbname=postgresscanner' AS s (TYPE POSTGRES)
+
+statement ok
+CREATE OR REPLACE TABLE s.large_tbl AS FROM range(100000) t(i)
+
+query I
+FROM s.large_tbl LIMIT 5
+----
+0
+1
+2
+3
+4
+
+query I
+FROM s.large_tbl LIMIT 5 OFFSET 5
+----
+5
+6
+7
+8
+9
+
+statement ok
+set explain_output='optimized_only'
+
+# limit is now not in DuckDB plan, but sent down to Postgres, this checks that
+
+query II
+EXPLAIN FROM s.large_tbl LIMIT 5;
+----
+logical_opt	<!REGEX>:.*LIMIT.*