利用DeepSeek编写能在DuckDB中读PostgreSQL表的表函数

前文实现了UDF和UDAF，还有一类函数是表函数，它放在From 子句中，返回一个集合。DuckDB中已有PostgreSQL插件，但我们可以用pqxx库实现一个简易的只读read_pg()表函数。
提示词如下：

请将libpqxx库集成到我们的程序，使它能对postgresql数据库操作，并把数据与duckdb打通，比如能在一个sql中访问pg和duckdb中的表的关联结果，先做一个简单的表函数read_pg(db,table)返回一个表，可以执行select * from read_pg(db,table)；select * from read_pg(db,table1) a,read_pg(db,table2) b where a.x=b.x;其中db是postgresql连接字符串，比如postgresql://user:secret@localhost/mydb，table是一个varchar

DeepSeek编写的代码一开始总是无限重复调用Function函数，后来经过添加

    // 执行已完成的处理if (state.execution_finished) {output.SetCardinality(0);return;}

后能正确输出postgresql某个表的数据了，加where 条件也能查出。另一个插曲是，DeepSeek编写的表函数注册代码总是不对，我从网上找了一个例子，虽然我的程序不是插件，却能调用ExtensionUtil::RegisterFunction来注册。

源代码如下

#include <pqxx/pqxx>
#include <memory>
#include <unordered_map>
#include <duckdb.hpp>#include "duckdb/function/function_set.hpp"
#include "duckdb/parser/parsed_data/create_aggregate_function_info.hpp"class PGConnectionPool {
private:std::unordered_map<std::string, std::shared_ptr<pqxx::connection>> connections;public:pqxx::connection& getConnection(const std::string& conn_str) {auto it = connections.find(conn_str);if (it == connections.end()) {auto conn = std::make_shared<pqxx::connection>(conn_str);connections[conn_str] = conn;return *conn;}return *(it->second);}
};static PGConnectionPool pg_pool;struct PGTableFunctionData : public duckdb::TableFunctionData {std::string conn_str;std::string table_name;duckdb::vector<duckdb::LogicalType> return_types;duckdb::vector<std::string> return_names;PGTableFunctionData(std::string conn_str, std::string table_name, duckdb::vector<duckdb::LogicalType> return_types,duckdb::vector<std::string> return_names): conn_str(std::move(conn_str)), table_name(std::move(table_name)),return_types(std::move(return_types)), return_names(std::move(return_names)) {}duckdb::unique_ptr<duckdb::FunctionData> Copy() const override {return duckdb::make_uniq<PGTableFunctionData>(conn_str, table_name, return_types, return_names);}bool Equals(const duckdb::FunctionData &other) const override {auto &other_data = other.Cast<PGTableFunctionData>();return conn_str == other_data.conn_str && table_name == other_data.table_name;}
};struct PGGlobalState : public duckdb::GlobalTableFunctionState {pqxx::connection* conn = nullptr;std::unique_ptr<pqxx::work> txn;pqxx::result result;pqxx::result::const_iterator it;bool initialized = false;bool execution_finished = false;
};struct PGTableFunction {static duckdb::TableFunction GetFunction() {return duckdb::TableFunction("read_pg",{duckdb::LogicalType::VARCHAR, duckdb::LogicalType::VARCHAR},Function,Bind,InitGlobal);}static duckdb::unique_ptr<duckdb::GlobalTableFunctionState> InitGlobal(duckdb::ClientContext &context,duckdb::TableFunctionInitInput &input) {return duckdb::make_uniq<PGGlobalState>();}static duckdb::unique_ptr<duckdb::FunctionData> Bind(duckdb::ClientContext &context,duckdb::TableFunctionBindInput &input,duckdb::vector<duckdb::LogicalType> &return_types,duckdb::vector<std::string> &return_names) {auto conn_str = input.inputs[0].GetValue<std::string>();auto table_name = input.inputs[1].GetValue<std::string>();try {auto& conn = pg_pool.getConnection(conn_str);pqxx::work txn(conn);auto r = txn.exec("SELECT column_name, data_type FROM information_schema.columns ""WHERE table_name = " + txn.quote(table_name) + " ORDER BY ordinal_position");for (const auto& row : r) {return_names.push_back(row[0].as<std::string>());std::string pg_type = row[1].as<std::string>();if (pg_type == "integer" || pg_type == "bigint") {return_types.push_back(duckdb::LogicalType::BIGINT);} else if (pg_type == "text" || pg_type == "varchar") {return_types.push_back(duckdb::LogicalType::VARCHAR);} else if (pg_type == "double precision") {return_types.push_back(duckdb::LogicalType::DOUBLE);} else if (pg_type == "boolean") {return_types.push_back(duckdb::LogicalType::BOOLEAN);} else {return_types.push_back(duckdb::LogicalType::VARCHAR);}}return duckdb::make_uniq<PGTableFunctionData>(conn_str, table_name, return_types, return_names);} catch (const std::exception& e) {throw std::runtime_error("PostgreSQL error: " + std::string(e.what()));}}static void Function(duckdb::ClientContext &context,duckdb::TableFunctionInput &data,duckdb::DataChunk &output) {auto &bind_data = data.bind_data->Cast<PGTableFunctionData>();auto &state = data.global_state->Cast<PGGlobalState>();if (state.execution_finished) {output.SetCardinality(0);return;}try {if (!state.initialized) {state.conn = &pg_pool.getConnection(bind_data.conn_str);state.txn = std::make_unique<pqxx::work>(*state.conn);state.result = state.txn->exec("SELECT * FROM " + state.txn->quote_name(bind_data.table_name));state.it = state.result.begin();state.initialized = true;}idx_t row_count = 0;while (state.it != state.result.end() && row_count < STANDARD_VECTOR_SIZE) {const auto& row = *state.it;for (duckdb::idx_t col = 0; col < static_cast<duckdb::idx_t>(row.size()); col++) {auto field = row[static_cast<pqxx::row::size_type>(col)];if (field.is_null()) {output.data[col].SetValue(row_count, duckdb::Value());} else {std::string value = field.as<std::string>();switch (bind_data.return_types[col].id()) {case duckdb::LogicalTypeId::BIGINT:output.data[col].SetValue(row_count, duckdb::Value::BIGINT(std::stoll(value)));break;case duckdb::LogicalTypeId::DOUBLE:output.data[col].SetValue(row_count, duckdb::Value::DOUBLE(std::stod(value)));break;case duckdb::LogicalTypeId::BOOLEAN:output.data[col].SetValue(row_count, duckdb::Value::BOOLEAN(value == "t" || value == "true"));break;default:output.data[col].SetValue(row_count, duckdb::Value(value));}}}row_count++;++state.it;}output.SetCardinality(row_count);if (state.it == state.result.end()) {state.execution_finished = true;state.txn->commit();}} catch (const std::exception& e) {//if (state.txn && state.txn->is_open()) state.txn->abort();throw std::runtime_error("PostgreSQL error: " + std::string(e.what()));}}
};

测试代码如下

#include "duckdb.hpp"
#include "readpg5.cpp"
#include <iostream>
#include "duckdb/main/extension_util.hpp"using namespace duckdb;
using namespace std;int main() {DuckDB db(nullptr);Connection con(db);try {DatabaseInstance& db_instance = *db.instance;ExtensionUtil::RegisterFunction(db_instance, PGTableFunction::GetFunction());} catch (const exception &e) {cerr << "初始化错误: " << e.what() << endl;return 1;}cout << "=== 测试1: 查询PostgreSQL表函数 ===" << endl;auto result = con.Query("SELECT * FROM read_pg('postgresql://postgres@127.0.0.1/postgres', 't2')");if (result->HasError()) {cerr << "查询错误: " << result->GetError() << endl;} else {result->Print();}cout << "\n=== 测试1.1: 带条件的同一个表查询 ===" << endl;result = con.Query("SELECT * FROM read_pg('postgresql://postgres@127.0.0.1/postgres', 't2') WHERE tid = 2");if (result->HasError()) {cerr << "查询错误: " << result->GetError() << endl;} else {result->Print();}cout << "\n=== 测试2: 带条件的查询 ===" << endl;result = con.Query("SELECT * FROM read_pg('postgresql://postgres@127.0.0.1/postgres', 't') WHERE a = 2");if (result->HasError()) {cerr << "查询错误: " << result->GetError() << endl;} else {result->Print();}cout << "\n=== 测试3: 多表查询 ===" << endl;result = con.Query("SELECT tname FROM read_pg('postgresql://postgres@127.0.0.1/postgres', 't2') a, ""read_pg('postgresql://postgres@127.0.0.1/postgres', 't') b WHERE a.tid = b.a");if (result->HasError()) {cerr << "查询错误: " << result->GetError() << endl;} else {result->Print();}cout << "\n=== 测试4: pg和duckdb多表查询 ===" << endl;con.Query("create table duckdb_t as select 2 a union all select 3");result = con.Query("SELECT tname FROM read_pg('postgresql://postgres@127.0.0.1/postgres', 't2') a, ""duckdb_t b WHERE a.tid = b.a");if (result->HasError()) {cerr << "查询错误: " << result->GetError() << endl;} else {result->Print();}cout << "\n=== 测试完成 ===" << endl;return 0;
}

编译命令行

export LIBRARY_PATH=/par/duck/build/src
export LD_LIBRARY_PATH=/par/duck/build/src
g++ -std=c++17 -o readpg2 testpg5.cpp -lduckdb -lpqxx -lpq -I /par/duck/src/include