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Transactions: MariaDB and PostgreSQL

Every file of the MariaDB and PostgreSQL versions side by side, with the lines that differ marked.

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Configuration

sgl.yamldiffers
MariaDB
PostgreSQL
1version: 1
1version: 1
2
2
3databases:
3databases:
4 - name: bank
4 - name: bank
5 dialect: mysql
5 dialect: postgres
6 migrations: db/migrations
6 migrations: db/migrations
7 queries: db/queries
7 queries: db/queries
8 go:
8 go:
9 package: store
9 package: store
10 out: internal/store
10 out: internal/store
11 driver: database/sql
12 interface:
13 enabled: true
14 migrate:
15 dsn: ${env:DATABASE_URL}

Migrations

db/migrations/0001_accounts.up.sqldiffers
MariaDB
PostgreSQL
1CREATE TABLE accounts (
1CREATE TABLE accounts (
2 id BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
2 id BIGINT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
3 owner VARCHAR(50) NOT NULL,
3 owner TEXT NOT NULL,
4 balance INT NOT NULL,
4 balance INTEGER NOT NULL CHECK (balance >= 0)
5 CONSTRAINT balance_not_negative CHECK (balance >= 0)
6);
5);
db/migrations/0001_accounts.down.sqlidentical
MariaDB
PostgreSQL
1DROP TABLE accounts;
1DROP TABLE accounts;

Queries

OpenAccountdiffers
MariaDB · db/queries/accounts.sql
PostgreSQL · db/queries/accounts.sql
1-- sgl.query(OpenAccount, execresult)
1-- sgl.query(OpenAccount, one)
2INSERT INTO accounts (owner, balance) VALUES (?, ?);
2INSERT INTO accounts (owner, balance) VALUES ($1, $2) RETURNING id;
Depositidentical
MariaDB · db/queries/accounts.sql
PostgreSQL · db/queries/accounts.sql
1-- sgl.query(Deposit, exec)
1-- sgl.query(Deposit, exec)
2UPDATE accounts SET balance = balance + :amount WHERE id = :id;
2UPDATE accounts SET balance = balance + :amount WHERE id = :id;
Withdrawidentical
MariaDB · db/queries/accounts.sql
PostgreSQL · db/queries/accounts.sql
1-- sgl.query(Withdraw, exec)
1-- sgl.query(Withdraw, exec)
2UPDATE accounts SET balance = balance - :amount WHERE id = :id;
2UPDATE accounts SET balance = balance - :amount WHERE id = :id;
ListAccountsidentical
MariaDB · db/queries/accounts.sql
PostgreSQL · db/queries/accounts.sql
1-- sgl.query(ListAccounts, many)
1-- sgl.query(ListAccounts, many)
2SELECT id, owner, balance FROM accounts ORDER BY id;
2SELECT id, owner, balance FROM accounts ORDER BY id;

What sgl generates

OpenAccountdiffers
MariaDB
PostgreSQL
1func (queries *Queries) OpenAccount(ctx context.Context, params OpenAccountParams) (sql.Result, error)
1func (queries *Queries) OpenAccount(ctx context.Context, params OpenAccountParams) (OpenAccountRow, error)
2
2
3type OpenAccountParams struct {
3type OpenAccountParams struct {
4 Owner string
4 Owner string
5 Balance int32
5 Balance int32
6}
6}
7
8type OpenAccountRow struct {
9 ID int64 `json:"id"`
10}
Depositidentical
MariaDB
PostgreSQL
1func (queries *Queries) Deposit(ctx context.Context, params DepositParams) error
1func (queries *Queries) Deposit(ctx context.Context, params DepositParams) error
2
2
3type DepositParams struct {
3type DepositParams struct {
4 Amount int32
4 Amount int32
5 ID int64
5 ID int64
6}
6}
Withdrawidentical
MariaDB
PostgreSQL
1func (queries *Queries) Withdraw(ctx context.Context, params WithdrawParams) error
1func (queries *Queries) Withdraw(ctx context.Context, params WithdrawParams) error
2
2
3type WithdrawParams struct {
3type WithdrawParams struct {
4 Amount int32
4 Amount int32
5 ID int64
5 ID int64
6}
6}
ListAccountsidentical
MariaDB
PostgreSQL
1func (queries *Queries) ListAccounts(ctx context.Context) ([]ListAccountsRow, error)
1func (queries *Queries) ListAccounts(ctx context.Context) ([]ListAccountsRow, error)
2
2
3type ListAccountsRow struct {
3type ListAccountsRow struct {
4 ID int64 `json:"id"`
4 ID int64 `json:"id"`
5 Owner string `json:"owner"`
5 Owner string `json:"owner"`
6 Balance int32 `json:"balance"`
6 Balance int32 `json:"balance"`
7}
7}

The program

example.godiffers
MariaDB
PostgreSQL
1package main
1package main
2
2
3import (
3import (
4 "context"
4 "context"
5 "database/sql"
5 "database/sql"
6 "errors"
6 "errors"
7 "fmt"
7 "fmt"
8 "io"
8 "io"
9 "strings"
10
9
11 "seagull.sh/seagull/examples/mysql/023_transactions/internal/store"
10 "seagull.sh/seagull/examples/postgres/023_transactions/internal/store"
12)
11)
13
12
13// sqlStateError is a server error that carries a SQLSTATE code, as pgx's *pgconn.PgError
14// does.
15type sqlStateError interface {
16 error
17
18 // SQLState returns the five-character SQLSTATE code.
19 SQLState() string
20}
21
14// run opens two accounts in a transaction it manages itself, then makes two transfers
22// run opens two accounts in a transaction it manages itself, then makes two transfers
15// with RunInTx: one commits, and one breaks the balance check part-way and rolls back.
23// with RunInTx: one commits, and one breaks the balance check part-way and rolls back.
16//
24//
25// sgl.yaml sets go.interface, so the generated package also declares store.Querier, an
26// interface of every query method, which *store.Queries implements whether it runs on the
27// database or inside a transaction. printBalances takes a store.Querier, so the same
28// function reads the committed balances and, inside each transfer, the balances the
29// transaction sees before it commits or rolls back; a test could pass it a fake instead.
30//
17// Takes database (*sql.DB) which is a migrated database.
31// Takes database (*sql.DB) which is a migrated database.
18// Takes output (io.Writer) which receives each step and the balances.
32// Takes output (io.Writer) which receives each step and the balances.
19//
33//
20// Returns error when a query fails other than as the example expects.
34// Returns error when a query fails other than as the example expects.
21func run(ctx context.Context, database *sql.DB, output io.Writer) error {
35func run(ctx context.Context, database *sql.DB, output io.Writer) error {
22 queries := store.New(database)
36 queries := store.New(database)
23
37
24 pip, gull, err := openAccounts(ctx, database, queries)
38 pip, gull, err := openAccounts(ctx, database, queries)
25 if err != nil {
39 if err != nil {
26 return fmt.Errorf("opening accounts: %w", err)
40 return fmt.Errorf("opening accounts: %w", err)
27 }
41 }
28 fmt.Fprintln(output, "opened accounts for Pip and Gull: committed")
42 fmt.Fprintln(output, "opened accounts for Pip and Gull: committed")
29 if err := printBalances(ctx, queries, output); err != nil {
43 if err := printBalances(ctx, queries, output, "balances"); err != nil {
30 return err
44 return err
31 }
45 }
32
46
33 if err := transfer(ctx, database, queries, pip, gull, 1500); err != nil {
47 if err := transfer(ctx, database, queries, output, pip, gull, 1500); err != nil {
34 return fmt.Errorf("first transfer: %w", err)
48 return fmt.Errorf("first transfer: %w", err)
35 }
49 }
36 fmt.Fprintln(output, "transferred 1500 from Pip to Gull: committed")
50 fmt.Fprintln(output, "transferred 1500 from Pip to Gull: committed")
37 if err := printBalances(ctx, queries, output); err != nil {
51 if err := printBalances(ctx, queries, output, "balances"); err != nil {
38 return err
52 return err
39 }
53 }
40
54
41 // Gull holds 3500, so this transfer credits Pip and then fails the CHECK constraint when
55 // Gull holds 3500, so this transfer credits Pip and then fails the CHECK constraint when
42 // it debits Gull. RunInTx rolls back, and the credit to Pip that had already run is
56 // it debits Gull. RunInTx rolls back, and the credit to Pip that had already run is
43 // undone with it. MySQL reports the failure as error 3819, naming the constraint.
57 // undone with it.
44 // MariaDB words it differently and names the database too, so the example checks for the
58 err = transfer(ctx, database, queries, output, gull, pip, 9000)
45 // constraint's name and prints its own line.
46 err = transfer(ctx, database, queries, gull, pip, 9000)
47 if err == nil {
59 if err == nil {
48 return errors.New("an overdrawing transfer committed")
60 return errors.New("an overdrawing transfer committed")
49 }
61 }
50 if !strings.Contains(err.Error(), "balance_not_negative") {
62
51 return fmt.Errorf("second transfer failed other than at the balance check: %w", err)
63 // pgx reports a server error as a *pgconn.PgError, whose SQLState method gives the
64 // standard five-character code: 23514 for a failed CHECK constraint. Match on the code,
65 // not the message, as the wording differs between servers and versions.
66 failure, hasState := errors.AsType[sqlStateError](err)
67 if !hasState {
68 return fmt.Errorf("an overdrawing transfer failed without a SQLSTATE: %w", err)
52 }
69 }
53 fmt.Fprintln(output, "transferred 9000 from Gull to Pip: rolled back, as the debit broke balance_not_negative")
70 fmt.Fprintf(output, "transferred 9000 from Gull to Pip: rolled back (SQLSTATE %s)\n", failure.SQLState())
54 return printBalances(ctx, queries, output)
71 return printBalances(ctx, queries, output, "balances")
55}
72}
56
73
57// openAccounts opens both accounts in one transaction that it begins and commits itself.
74// openAccounts opens both accounts in one transaction that it begins and commits itself.
58// WithTx returns a copy of queries bound to that transaction, which suits a transaction
75// WithTx returns a copy of queries bound to that transaction, which suits a transaction
59// begun with options or shared with code that does not use Seagull.
76// begun with options or shared with code that does not use Seagull.
60//
77//
61// Takes database (*sql.DB) which begins the transaction.
78// Takes database (*sql.DB) which begins the transaction.
62// Takes queries (*store.Queries) which are bound to it.
79// Takes queries (*store.Queries) which are bound to it.
63//
80//
64// Returns pipID (int64) which is Pip's account ID.
81// Returns pipID (int64) which is Pip's account ID.
65// Returns gullID (int64) which is Gull's account ID.
82// Returns gullID (int64) which is Gull's account ID.
66// Returns err (error) when an account cannot be opened or the commit fails.
83// Returns err (error) when an account cannot be opened or the commit fails.
67func openAccounts(ctx context.Context, database *sql.DB, queries *store.Queries) (pipID, gullID int64, err error) {
84func openAccounts(ctx context.Context, database *sql.DB, queries *store.Queries) (pipID, gullID int64, err error) {
68 transaction, err := database.BeginTx(ctx, nil)
85 transaction, err := database.BeginTx(ctx, nil)
69 if err != nil {
86 if err != nil {
70 return 0, 0, err
87 return 0, 0, err
71 }
88 }
72 // Rollback after a successful Commit does nothing, so deferring it covers every early
89 // Rollback after a successful Commit does nothing, so deferring it covers every early
73 // return.
90 // return.
74 defer func() { _ = transaction.Rollback() }()
91 defer func() { _ = transaction.Rollback() }()
75
92
76 inTransaction := queries.WithTx(transaction)
93 inTransaction := queries.WithTx(transaction)
77 pip, err := openAccount(ctx, inTransaction, "Pip", 5000)
94 pip, err := inTransaction.OpenAccount(ctx, store.OpenAccountParams{Owner: "Pip", Balance: 5000})
78 if err != nil {
95 if err != nil {
79 return 0, 0, err
96 return 0, 0, err
80 }
97 }
81 gull, err := openAccount(ctx, inTransaction, "Gull", 2000)
98 gull, err := inTransaction.OpenAccount(ctx, store.OpenAccountParams{Owner: "Gull", Balance: 2000})
82 if err != nil {
99 if err != nil {
83 return 0, 0, err
100 return 0, 0, err
84 }
101 }
85 return pip, gull, transaction.Commit()
102 return pip.ID, gull.ID, transaction.Commit()
86}
87
88// openAccount opens an account and returns the id MySQL gave it. MySQL has no RETURNING,
89// so OpenAccount is execresult, and LastInsertId reads the AUTO_INCREMENT id from the
90// result of the INSERT, on the transaction's own connection.
91//
92// Takes queries (*store.Queries) which run the INSERT.
93// Takes owner (string) which names the account holder.
94// Takes balance (int32) which is the opening balance, in pence.
95//
96// Returns int64 which is the new account's ID.
97// Returns error when the INSERT fails.
98func openAccount(ctx context.Context, queries *store.Queries, owner string, balance int32) (int64, error) {
99 result, err := queries.OpenAccount(ctx, store.OpenAccountParams{Owner: owner, Balance: balance})
100 if err != nil {
101 return 0, err
102 }
103 return result.LastInsertId()
104}
103}
105
104
106// transfer moves an amount between two accounts with RunInTx, which begins a transaction,
105// transfer moves an amount between two accounts with RunInTx, which begins a transaction,
107// passes fn a Queries bound to it, and commits when fn returns nil or rolls back when it
106// passes fn a Queries bound to it, and commits when fn returns nil or rolls back when it
108// returns an error.
107// returns an error. After the credit it prints the balances the transaction sees, through
108// the same printBalances the caller uses.
109//
109//
110// Takes database (*sql.DB) which begins the transaction.
110// Takes database (*sql.DB) which begins the transaction.
111// Takes queries (*store.Queries) which run the transfer.
111// Takes queries (*store.Queries) which run the transfer.
112// Takes output (io.Writer) which receives the balances inside the transaction.
112// Takes from (int64) which is the account to debit.
113// Takes from (int64) which is the account to debit.
113// Takes to (int64) which is the account to credit.
114// Takes to (int64) which is the account to credit.
114// Takes amount (int32) which is the sum to move, in pence.
115// Takes amount (int32) which is the sum to move, in pence.
115//
116//
116// Returns error when either update fails; the transaction is then rolled back.
117// Returns error when either update fails; the transaction is then rolled back.
117func transfer(ctx context.Context, database *sql.DB, queries *store.Queries, from, to int64, amount int32) error {
118func transfer(ctx context.Context, database *sql.DB, queries *store.Queries, output io.Writer, from, to int64, amount int32) error {
118 return queries.RunInTx(ctx, database, func(inTransaction *store.Queries) error {
119 return queries.RunInTx(ctx, database, func(inTransaction *store.Queries) error {
119 if err := inTransaction.Deposit(ctx, store.DepositParams{Amount: amount, ID: to}); err != nil {
120 if err := inTransaction.Deposit(ctx, store.DepositParams{Amount: amount, ID: to}); err != nil {
120 return err
121 return err
121 }
122 }
123 if err := printBalances(ctx, inTransaction, output, "inside the transaction"); err != nil {
124 return err
125 }
122 return inTransaction.Withdraw(ctx, store.WithdrawParams{Amount: amount, ID: from})
126 return inTransaction.Withdraw(ctx, store.WithdrawParams{Amount: amount, ID: from})
123 })
127 })
124}
128}
125
129
126// printBalances writes every account's balance on one line.
130// printBalances writes every account's balance on one line. It takes the generated
131// store.Querier rather than *store.Queries, so it reads through whatever it is given: the
132// queries on the database, the queries bound to a transaction, or a fake in a test.
127//
133//
128// Takes queries (*store.Queries) which read the accounts.
134// Takes queries (store.Querier) which read the accounts.
129// Takes output (io.Writer) which receives the line.
135// Takes output (io.Writer) which receives the line.
136// Takes label (string) which says whose view of the balances the line shows.
130//
137//
131// Returns error when the accounts cannot be listed.
138// Returns error when the accounts cannot be listed.
132func printBalances(ctx context.Context, queries *store.Queries, output io.Writer) error {
139func printBalances(ctx context.Context, queries store.Querier, output io.Writer, label string) error {
133 accounts, err := queries.ListAccounts(ctx)
140 accounts, err := queries.ListAccounts(ctx)
134 if err != nil {
141 if err != nil {
135 return fmt.Errorf("listing accounts: %w", err)
142 return fmt.Errorf("listing accounts: %w", err)
136 }
143 }
137 fmt.Fprint(output, " balances:")
144 fmt.Fprintf(output, " %s:", label)
138 for _, account := range accounts {
145 for _, account := range accounts {
139 fmt.Fprintf(output, " %s %d", account.Owner, account.Balance)
146 fmt.Fprintf(output, " %s %d", account.Owner, account.Balance)
140 }
147 }
141 fmt.Fprintln(output)
148 fmt.Fprintln(output)
142 return nil
149 return nil
143}
150}