FlexQL

A simplified SQL-like in-memory database driver implemented in C++17.# FlexQL

Features

Feature	Details
Types	INT, DECIMAL, VARCHAR, DATETIME
Commands	CREATE TABLE, INSERT INTO, SELECT, WHERE, INNER JOIN
WHERE operators	= != < > <= >=
Row expiry	INSERT ... TTL <seconds>
Indexing	BST primary index on first column — O(log n) equality lookup
Caching	LRU cache (1024 entries) — identical SELECTs served instantly
Concurrency	One thread per client, mutex-protected engine

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Quick Start

1. Build

make

Produces bin/flexql-server and bin/flexql-client.

2. Start the server

./bin/flexql-server 9000

3. Connect a client (new terminal)

./bin/flexql-client 127.0.0.1 9000

4. Run SQL

CREATE TABLE STUDENT(ID INT, NAME VARCHAR);
INSERT INTO STUDENT VALUES (1,'Alice');
INSERT INTO STUDENT VALUES (2,'Bob');
SELECT * FROM STUDENT;
SELECT * FROM STUDENT WHERE ID > 1;
.exit

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Supported SQL Syntax

CREATE TABLE

CREATE TABLE table_name (col1 TYPE, col2 TYPE, ...);

Supported types: INT, DECIMAL, VARCHAR, DATETIME

INSERT

INSERT INTO table_name VALUES (val1, val2, ...);
INSERT INTO table_name VALUES (val1, val2, ...) TTL 3600;  -- expires in 1 hour

SELECT

SELECT * FROM table_name;
SELECT col1, col2 FROM table_name;
SELECT * FROM table_name WHERE col = value;
SELECT * FROM t1 INNER JOIN t2 ON t1.col = t2.col;
SELECT * FROM t1 INNER JOIN t2 ON t1.col = t2.col WHERE t2.col2 = 'x';

DATETIME values

INSERT INTO events VALUES (1, 'Launch', '2024-03-15 09:00:00');
SELECT * FROM events WHERE ts > '2024-01-01';

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Running the Tests

make test

Starts a temporary server on port 9099, runs 42 automated tests, then stops the server.

Expected output:

Results: 42 passed, 0 failed

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Performance Benchmark

./bin/benchmark_flexql 1000000

Measures INSERT throughput, full scan, index lookup, range scan, DATETIME range, VARCHAR scan, and LRU cache hit time. Reports peak server RSS memory.

Change the third argument to 10000000 for a 10-million-row benchmark.

---

C API

#include "include/flexql.h"

// Callback: invoked once per result row
int my_callback(void *data, int nCols, char **values, char **colNames) {
    for (int i = 0; i < nCols; i++)
        printf("%s = %s\n", colNames[i], values[i]);
    printf("\n");
    return 0;  // return 1 to abort early
}

int main() {
    FlexQL *db;
    if (flexql_open("127.0.0.1", 9000, &db) != FLEXQL_OK) return 1;

    char *err = NULL;
    flexql_exec(db, "CREATE TABLE T(ID INT, NAME VARCHAR);", NULL, NULL, &err);
    flexql_exec(db, "INSERT INTO T VALUES (1,'Alice');",    NULL, NULL, &err);
    flexql_exec(db, "SELECT * FROM T;", my_callback, NULL, &err);

    if (err) { fprintf(stderr, "%s\n", err); flexql_free(err); }
    flexql_close(db);
}

Compile against the API:

g++ -std=c++17 -I include -o myapp myapp.cpp src/client/flexql_api.cpp

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Project Structure

flexql/
├── Makefile
├── README.md
├── DESIGN.md               — design decisions, algorithms, complexity table
├── include/
│   ├── flexql.h            — public C API
│   ├── common/types.h      — ColType, CellValue, Row, Table, Database
│   ├── parser/lexer.h      — token types
│   ├── parser/parser.h     — TokenStream, Condition, JoinCondition
│   ├── query/engine.h      — Engine class
│   ├── query/result.h      — QueryResult struct
│   ├── cache/lru_cache.h   — LRUCache
│   ├── network/protocol.h  — wire protocol constants
│   ├── expiration/ttl.h    — isExpired()
│   ├──storage/storage.h     Header file storae
│   └── concurrency/        — MutexGuard RAII wrapper
├── src/
│   ├── parser/lexer.cpp
│   ├── parser/parser.cpp
│   ├── cache/lru_cache.cpp
│   ├── query/engine.cpp    — CREATE / INSERT / SELECT / INNER JOIN
│   ├── storage/storage.cpp   storage implementation
│   ├── server/server.cpp   — TCP server only
│   └── client/
│       ├── flexql_api.cpp  — C API implementation (no main)
│       └── client.cpp      — interactive REPL main
├── tests/
│   └── test_all.cpp        — 42 automated tests
└── scripts/
    └── benchmark.sh        — performance benchmark