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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchMAX(invoice_number) + 1 can produce duplicate invoice numbers because it reads the current maximum and calculates a value without reserving that value atomically. Two concurrent transactions may read the same maximum and both calculate the same next number. A database uniqueness constraint is what prevents both duplicate rows from committing; a sequence or a serialized counter is what determines how numbers should be allocated.
How the MAX()+1 race happens
Suppose the highest committed invoice number is 410. Session A reads 410 and calculates 411. Before A inserts its invoice, session B also reads 410 and calculates 411. Both have made a valid calculation from the data they observed, but neither calculation reserved 411.
This is a concurrency race: the result depends on transaction timing, isolation level, and locking behavior, so duplicates are possible rather than inevitable on every run. PostgreSQL’s concurrency documentation explains how simultaneous sessions, transaction isolation, and locking affect access to shared data: PostgreSQL 15: Concurrency Control.
How to prevent duplicate invoice numbers
Enforce uniqueness in the database
Add a unique constraint or unique index for the actual business scope of the number. If invoice numbers must be unique across the whole database, enforce uniqueness on the number. If they are unique only within a tenant, year, or document series, enforce uniqueness on the combination of that scope and the number.
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This protects data integrity even when application code races: at most one conflicting insert can succeed. The other transaction must handle the uniqueness error, for example by retrying with a newly allocated value when that fits the chosen numbering policy. The constraint prevents duplicate committed rows, but it does not itself make allocation smooth or gapless. See the database-specific guidance on SQL Server unique constraints.
Choose an allocation strategy for your requirements
Decide whether gaps are acceptable, whether a number must be known before an invoice is inserted, whether numbering is shared across tables, and whether numbering is global or scoped by tenant, year, or series. The right mechanism depends on those requirements.
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| Requirement | Suitable approach | Important trade-off |
|---|---|---|
| Gaps are acceptable; each row needs a distinct value | Database identity or sequence, plus a unique constraint | Allocated values can be lost after rollbacks, failed inserts, or unused allocations; these generators do not by themselves enforce uniqueness. |
| A number is needed before insert or shared across tables | A database sequence, plus a unique constraint where uniqueness is required | A sequence value is allocated when requested, whether or not it is ultimately inserted. |
| A committed series must be gapless | A counter row for each numbering scope, updated and consumed in the same transaction as the invoice insert | Allocations must be serialized, which creates contention; validate locking, isolation, and failure handling on the actual database engine. |
When gaps are acceptable: identity or sequence
Use the engine’s identity or sequence facility rather than deriving a number from the current maximum. Keep a primary key or unique constraint if the generated value must be unique: generation and uniqueness enforcement are separate responsibilities. SQL Server documents that identity values can differ across concurrent transactions but do not themselves guarantee uniqueness; sequences likewise do not enforce uniqueness. See SQL Server sequence objects and SQL Server IDENTITY property.
When a number is needed before insert or shared across tables
A sequence can supply a value before an invoice row is inserted, and the same sequence can serve multiple tables. That is useful when another operation needs the number in advance or when several tables must draw from one series. The allocation is not rolled back simply because the value is never inserted, so gaps are possible. Microsoft states, “Uniqueness isn’t automatically enforced for sequence values.” If sequence values in a table must be unique, create a unique constraint on the column.
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When the committed series must be gapless
Identity and sequence values are generally a poor fit when the requirement is a gapless series of committed invoices. Microsoft documents gaps from unused or rolled-back sequence allocations and from failed or rolled-back identity inserts. A common alternative is a counter row per numbering scope: increment the counter and insert the invoice in one transaction, with competing allocations serialized. Test the transaction and locking behavior on the deployed engine, including what happens when an insert fails or a transaction rolls back.
Do not assume that an ordinary auto-number feature creates a gapless, commit-ordered legal invoice sequence. Whether invoice numbering has legal or accounting requirements depends on jurisdiction; the database mechanisms described here do not establish those rules.
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Keep database details engine-specific
The concurrency principle applies broadly, but syntax and behavior vary by database engine and release. The concurrency explanation above cites PostgreSQL 15 documentation; the identity and sequence trade-offs cite SQL Server documentation. Check the documentation for the exact engine and version you deploy rather than copying SQL or assumptions across systems.
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