How do savepoints and constraint failures behave inside a transaction, and when do you use FTS5 instead of LIKE?

Predict first

What do savepoints, constraint failures and FTS5 do?

import Foundation
import SQLite3

final class DB {
    var h: OpaquePointer?
    init(_ path: String = ":memory:", busy: Int32 = 0) { sqlite3_open(path, &h); sqlite3_busy_timeout(h, busy) }
    deinit { sqlite3_close(h) }
    @discardableResult func exec(_ sql: String) -> String? {
        var err: UnsafeMutablePointer<CChar>?
        if sqlite3_exec(h, sql, nil, nil, &err) != SQLITE_OK { defer { sqlite3_free(err) }; return String(cString: err!) }
        return nil
    }
    func rows(_ sql: String) -> [[String]] {
        var st: OpaquePointer?; var out: [[String]] = []
        guard sqlite3_prepare_v2(h, sql, -1, &st, nil) == SQLITE_OK else { return [["error: " + String(cString: sqlite3_errmsg(h))]] }
        defer { sqlite3_finalize(st) }
        while sqlite3_step(st) == SQLITE_ROW {
            out.append((0..<sqlite3_column_count(st)).map { i in sqlite3_column_text(st, i).map { String(cString: $0) } ?? "NULL" })
        }
        return out
    }
}

let db = DB()
db.exec("CREATE TABLE t(v INTEGER)")
db.exec("BEGIN"); db.exec("INSERT INTO t VALUES(1)")
db.exec("SAVEPOINT sp"); db.exec("INSERT INTO t VALUES(2)")
db.exec("ROLLBACK TO sp"); db.exec("RELEASE sp")
db.exec("COMMIT")
print("rows after savepoint rollback:", db.rows("SELECT v FROM t").map { $0[0] })
db.exec("CREATE TABLE u(id INTEGER PRIMARY KEY, name TEXT NOT NULL CHECK(length(name) > 1))")
print("not null:", db.exec("INSERT INTO u(name) VALUES(NULL)") ?? "ok")
print("check:", db.exec("INSERT INTO u(name) VALUES('x')") ?? "ok")
db.exec("BEGIN")
print("failing statement inside a transaction:", db.exec("INSERT INTO u(name) VALUES('x')") ?? "ok")
db.exec("INSERT INTO u(name) VALUES('ok')")
db.exec("COMMIT")
print("committed rows:", db.rows("SELECT name FROM u").map { $0[0] })
db.exec("CREATE VIRTUAL TABLE doc USING fts5(body)")
db.exec("INSERT INTO doc VALUES('swift concurrency actors'), ('combine publishers'), ('actors and isolation')")
print("fts match:", db.rows("SELECT body FROM doc WHERE doc MATCH 'actors' ORDER BY rowid").map { $0[0] })
print("fts prefix:", db.rows("SELECT body FROM doc WHERE doc MATCH 'pub*'").map { $0[0] })

• rows after savepoint rollback: ["1"] / constraint errors / committed rows: ["ok"] / fts match returns both "actors" rows / prefix matches "combine publishers"
• rows after savepoint rollback: ["1", "2"]
• committed rows: []