# Limbo Database Manual
Welcome to Limbo database manual!
## Table of contents
* [Introduction](#introduction)
* [Getting Started](#getting-started)
* [Limitations](#limitations)
* [The SQL language](#the-sql-language)
* [`ALTER TABLE` — change table definition](#alter-table--change-table-definition)
* [`BEGIN TRANSACTION` — start a transaction](#begin-transaction--start-a-transaction)
* [`COMMIT TRANSACTION` — commit the current transaction](#commit-transaction--commit-the-current-transaction)
* [`CREATE INDEX` — define a new index](#create-index--define-a-new-index)
* [`CREATE TABLE` — define a new table](#create-table--define-a-new-table)
* [`DELETE` - delete rows from a table](#delete---delete-rows-from-a-table)
* [`DROP INDEX` - remove an index](#drop-index---remove-an-index)
* [`DROP TABLE` — remove a table](#drop-table--remove-a-table)
* [`END TRANSACTION` — commit the current transaction](#end-transaction--commit-the-current-transaction)
* [`INSERT` — create new rows in a table](#insert--create-new-rows-in-a-table)
* [`ROLLBACK TRANSACTION` — abort the current transaction](#rollback-transaction--abort-the-current-transaction)
* [`SELECT` — retrieve rows from a table](#select--retrieve-rows-from-a-table)
* [`UPDATE` — update rows of a table](#update--update-rows-of-a-table)
* [SQLite C API](#sqlite-c-api)
* [WAL manipulation](#wal-manipulation)
* [`libsql_wal_frame_count`](#libsql_wal_frame_count)
* [SQL Commands](#sql-commands)
* [Appendix A: Limbo Internals](#appendix-a-limbo-internals)
* [Frontend](#frontend)
* [Parser](#parser)
* [Code generator](#code-generator)
* [Query optimizer](#query-optimizer)
* [Virtual Machine](#virtual-machine)
* [Pager](#pager)
* [I/O](#io)
## Introduction
Limbo is an in-process relational database engine, aiming towards full compatibility with SQLite.
Unlike client-server database systems such as PostgreSQL or MySQL, which require applications to communicate over network protocols for SQL execution,
an in-process database is in your application memory space.
This embedded architecture eliminates network communication overhead, allowing for the best case of low read and write latencies in the order of sub-microseconds.
### Getting Started
You can install Limbo on your computer as follows:
```
curl --proto '=https' --tlsv1.2 -LsSf \
https://github.com/tursodatabase/turso/releases/latest/download/limbo-installer.sh | sh
```
When you have the software installed, you can start a SQL shell as follows:
```console
$ limbo
Limbo
Enter ".help" for usage hints.
Connected to a transient in-memory database.
Use ".open FILENAME" to reopen on a persistent database
limbo> SELECT 'hello, world';
hello, world
```
## Limitations
Limbo aims towards full SQLite compatibility but has the following limitations:
* No multi-process access
* No multi-threading
* No indexing
* No savepoints
* No triggers
* No views
* No vacuum
For more detailed list of SQLite compatibility, please refer to [COMPAT.md](../COMPAT.md).
## The SQL language
### `ALTER TABLE` — change table definition
**Synopsis:**
```sql
ALTER TABLE old_name RENAME TO new_name
ALTER TABLE table_name ADD COLUMN column_name [ column_type ]
ALTER TABLE table_name DROP COLUMN column_name
```
**Example:**
```console
limbo> CREATE TABLE t(x);
limbo> .schema t;
CREATE TABLE t (x);
limbo> ALTER TABLE t ADD COLUMN y TEXT;
limbo> .schema t
CREATE TABLE t ( x , y TEXT );
limbo> ALTER TABLE t DROP COLUMN y;
limbo> .schema t
CREATE TABLE t ( x );
```
### `BEGIN TRANSACTION` — start a transaction
**Synopsis:**
```sql
BEGIN [ transaction_mode ] [ TRANSACTION ]
```
where `transaction_mode` is one of the following:
* `DEFERRED`
* `IMMEDIATE`
* `EXCLUSIVE`
**See also:**
* [END TRANSACTION](#end-transaction--commit-the-current-transaction)
### `COMMIT TRANSACTION` — commit the current transaction
**Synopsis:**
```sql
COMMIT [ TRANSACTION ]
```
**See also:**
* [END TRANSACTION](#end-transaction--commit-the-current-transaction)
### `CREATE INDEX` — define a new index
> [!NOTE]
> Indexes are currently experimental in Limbo and not enabled by default.
**Synopsis:**
```sql
CREATE INDEX [ index_name ] ON table_name ( column_name )
```
**Example:**
```
limbo> CREATE TABLE t(x);
limbo> CREATE INDEX t_idx ON t(x);
```
### `CREATE TABLE` — define a new table
**Synopsis:**
```sql
CREATE TABLE table_name ( column_name [ column_type ], ... )
```
**Example:**
```console
limbo> DROP TABLE t;
limbo> CREATE TABLE t(x);
limbo> .schema t
CREATE TABLE t (x);
```
### `DELETE` - delete rows from a table
**Synopsis:**
```sql
DELETE FROM table_name [ WHERE expression ]
```
**Example:**
```console
limbo> DELETE FROM t WHERE x > 1;
```
### `DROP INDEX` - remove an index
> [!NOTE]
> Indexes are currently experimental in Limbo and not enabled by default.
**Example:**
```console
limbo> DROP INDEX idx;
```
### `DROP TABLE` — remove a table
**Example:**
```console
limbo> DROP TABLE t;
```
### `END TRANSACTION` — commit the current transaction
```sql
END [ TRANSACTION ]
```
**See also:**
* `COMMIT TRANSACTION`
### `INSERT` — create new rows in a table
**Synopsis:**
```sql
INSERT INTO table_name [ ( column_name, ... ) ] VALUES ( value, ... ) [, ( value, ... ) ...]
```
**Example:**
```
limbo> INSERT INTO t VALUES (1), (2), (3);
limbo> SELECT * FROM t;
┌───┐
│ x │
├───┤
│ 1 │
├───┤
│ 2 │
├───┤
│ 3 │
└───┘
```
### `ROLLBACK TRANSACTION` — abort the current transaction
```sql
ROLLBACK [ TRANSACTION ]
```
### `SELECT` — retrieve rows from a table
**Synopsis:**
```sql
SELECT expression
[ FROM table-or-subquery ]
[ WHERE condition ]
[ GROU BY expression ]
```
**Example:**
```console
limbo> SELECT 1;
┌───┐
│ 1 │
├───┤
│ 1 │
└───┘
limbo> CREATE TABLE t(x);
limbo> INSERT INTO t VALUES (1), (2), (3);
limbo> SELECT * FROM t WHERE x >= 2;
┌───┐
│ x │
├───┤
│ 2 │
├───┤
│ 3 │
└───┘
```
### `UPDATE` — update rows of a table
**Synopsis:**
```sql
UPDATE table_name SET column_name = value [WHERE expression]
```
**Example:**
```console
limbo> CREATE TABLE t(x);
limbo> INSERT INTO t VALUES (1), (2), (3);
limbo> SELECT * FROM t;
┌───┐
│ x │
├───┤
│ 1 │
├───┤
│ 2 │
├───┤
│ 3 │
└───┘
limbo> UPDATE t SET x = 4 WHERE x >= 2;
limbo> SELECT * FROM t;
┌───┐
│ x │
├───┤
│ 1 │
├───┤
│ 4 │
├───┤
│ 4 │
└───┘
```
## SQLite C API
Limbo supports the SQLite C API, with libSQL extensions.
### WAL manipulation
#### `libsql_wal_frame_count`
Get the number of frames in the WAL.
**Synopsis:**
```c
int libsql_wal_frame_count(sqlite3 *db, uint32_t *p_frame_count);
```
**Description:**
The `libsql_wal_frame_count` function returns the number of frames in the WAL
in the `p_frame_count` parameter.
**Return Values:**
* `SQLITE_OK` if the number of frames in the WAL file is successfully returned.
* `SQLITE_MISUSE` if the `db` is NULL.
* SQLITE_ERROR if an error occurs while getting the number of frames in the WAL
file.
**Safety Requirements:**
* The `db` parameter must be a valid pointer to a `sqlite3` database
connection.
* The `p_frame_count` must be a valid pointer to a `u32` that will store the
* number of frames in the WAL file.
## SQL Commands
## Appendix A: Limbo Internals
Limbo's architecture resembles SQLite's but differs primarily in its
asynchronous I/O model. This asynchronous design enables applications to
leverage modern I/O interfaces like `io_uring,` maximizing storage device
performance. While an in-process database offers significant performance
advantages, integration with cloud services remains crucial for operations
like backups. Limbo's asynchronous I/O model facilitates this by supporting
networked storage capabilities.
The high-level interface to Limbo is the same as in SQLite:
* SQLite query language
* The `sqlite3_prepare()` function for translating SQL statements to programs
("prepared statements")
* The `sqlite3_step()` function for executing programs
If we start with the SQLite query language, you can use the `limbo`
command, for example, to evaluate SQL statements in the shell:
```
limbo> SELECT 'hello, world';
hello, world
```
To execute this SQL statement, the shell uses the `sqlite3_prepare()`
interface to parse the statement and generate a bytecode program, a step
called preparing a statement. When a statement is prepared, it can be executed
using the `sqlite3_step()` function.
To illustrate the different components of Limbo, we can look at the sequence
diagram of a query from the CLI to the bytecode virtual machine (VDBE):
```mermaid
sequenceDiagram
participant main as cli/main
participant Database as core/lib/Database
participant Connection as core/lib/Connection
participant Parser as sql/mod/Parser
participant translate as translate/mod
participant Statement as core/lib/Statement
participant Program as vdbe/mod/Program
main->>Database: open_file
Database->>main: Connection
main->>Connection: query(sql)
Note left of Parser: Parser uses vendored sqlite3-parser
Connection->>Parser: next()
Note left of Parser: Passes the SQL query to Parser
Parser->>Connection: Cmd::Stmt (ast/mod.rs)
Note right of translate: Translates SQL statement into bytecode
Connection->>translate:translate(stmt)
translate->>Connection: Program
Connection->>main: Ok(Some(Rows { Statement }))
note right of main: a Statement with
a reference to Program is returned
main->>Statement: step()
Statement->>Program: step()
Note left of Program: Program executes bytecode instructions
See https://www.sqlite.org/opcode.html
Program->>Statement: StepResult
Statement->>main: StepResult
```
To drill down into more specifics, we inspect the bytecode program for a SQL
statement using the `EXPLAIN` command in the shell. For our example SQL
statement, the bytecode looks as follows:
```
limbo> EXPLAIN SELECT 'hello, world';
addr opcode p1 p2 p3 p4 p5 comment
---- ----------------- ---- ---- ---- ------------- -- -------
0 Init 0 4 0 0 Start at 4
1 String8 0 1 0 hello, world 0 r[1]='hello, world'
2 ResultRow 1 1 0 0 output=r[1]
3 Halt 0 0 0 0
4 Transaction 0 0 0 0
5 Goto 0 1 0 0
```
The instruction set of the virtual machine consists of domain specific
instructions for a database system. Every instruction consists of an
opcode that describes the operation and up to 5 operands. In the example
above, execution starts at offset zero with the `Init` instruction. The
instruction sets up the program and branches to a instruction at address
specified in operand `p2`. In our example, address 4 has the
`Transaction` instruction, which begins a transaction. After that, the
`Goto` instruction then branches to address 1 where we load a string
constant `'hello, world'` to register `r[1]`. The `ResultRow` instruction
produces a SQL query result using contents of `r[1]`. Finally, the
program terminates with the `Halt` instruction.
### Frontend
#### Parser
The parser is the module in the front end that processes SQLite query language input data, transforming it into an abstract syntax tree (AST) for further processing. The parser is an in-tree fork of [lemon-rs](https://github.com/gwenn/lemon-rs), which in turn is a port of SQLite parser into Rust. The emitted AST is handed over to the code generation steps to turn the AST into virtual machine programs.
#### Code generator
The code generator module takes AST as input and produces virtual machine programs representing executable SQL statements. At high-level, code generation works as follows:
1. `JOIN` clauses are transformed into equivalent `WHERE` clauses, which simplifies code generation.
2. `WHERE` clauses are mapped into bytecode loops
3. `ORDER BY` causes the bytecode program to pass result rows to a sorter before returned to the application.
4. `GROUP BY` also causes the bytecode programs to pass result rows to an aggregation function before results are returned to the application.
#### Query optimizer
### Virtual Machine
### Pager
TODO
### I/O
TODO
[SQLite]: https://www.sqlite.org/