Lesson 40 · Loading from disk - Build a SQL Database

Loading from disk

The final piece closes the loop - reading a saved database back into memory so your data survives a restart. Today you load the file and prove a full round trip.

The goal

Load a database from a saved file and confirm a save-then-load round trip reproduces the original data.

Start here - the target

TO DO

Scenario: Round-tripping the database through disk

Givena database with users(id INTEGER, name TEXT) holding [1, "alice"] and [2, "bob"], saved to a file

Whena fresh database is loaded from that file

Thenit has a users table with the same schema and the same two rows in order

Andquerying "SELECT name FROM users WHERE id = 2" on the loaded database returns ["bob"]

Background

Loading reverses lesson 39: read the file line by line, and rebuild the database - a table header creates a table, a column line extends its schema, a row line inserts values reconstructed from their kind tags so an integer comes back an integer and a NULL comes back a NULL. A save-then-load round trip must reproduce the original tables exactly, and a query against the loaded database must return the same answers as against the original.

Wire load-on-startup and save-on-exit into the REPL and the walking skeleton becomes a real, persistent SQL database: create tables, insert and query data, join and aggregate across tables, mutate rows, quit, restart, and find your data waiting. You have built - from a single typed value - a tokenizer, a recursive-descent parser, a relational execution engine, and durable storage. That storage survives a clean restart - but not yet a crash mid-save, which can still tear the file or lose the mutations since the last save. The next chapter closes that gap and makes persistence genuinely crash-safe.

Make it work

// parse the file format from lesson 39 back into tables:
//   TABLE -> create table; COL -> add column; ROW -> insert values
// rebuild each Value from its kind tag so types are restored
// wire the REPL to load on startup and save on exit

CheckpointDONE

Your database saves and loads its data, so a full save-then-load round trip survives a clean restart. Next you make that persistence crash-safe. Commit and stop here.