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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →MAX(number) + 1 can assign the same invoice number twice because two transactions may read the same maximum before either inserts a row. In PostgreSQL, a unique constraint can stop both copies from being committed, but it cannot generate the next number or guarantee a gapless series. Choose the numbering method based on whether you need unique values or a gapless committed sequence.
Why does MAX() + 1 give duplicate numbers?
It is a read-then-write race. Suppose a tenant’s highest invoice number is 41. Two requests run at nearly the same time:
- Transaction A reads
MAX(number)and gets 41. - Transaction B reads the same maximum and also gets 41.
- Both calculate 42 and try to insert it.
At PostgreSQL’s default isolation level, ordinary reads do not reserve the value they return. Unless the application uses additional coordination, both transactions can proceed with the same result. A unique constraint on the business key—for example, (tenant_id, number)—prevents duplicate rows from being committed, but one request then fails rather than receiving a safely allocated next number.
In an eight-session, 15-second pgbench test on PostgreSQL 17.10, Now-Next reported that default-isolation MAX(number) + 1 issued a previously used number for 140,977 of 161,479 rows. Those rows contained 20,502 distinct values; the article described 87% of invoices in that test as duplicates. These are the publisher’s measurements on one machine with default settings, not a general duplicate rate or a benchmark independently replicated. The test did not cover crashes, replication, or more than eight sessions. See Chris van Eijk’s Now-Next article.
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Uniqueness and gaplessness are different requirements
Uniqueness means no two issued invoices in the relevant series share a number. Gaplessness means the committed series has no missing numbers. A constraint can enforce uniqueness among stored rows; it cannot ensure that every number in a series is present. A numbering mechanism may provide unique values while leaving gaps after a failed transaction.
The legal and administrative requirement depends on jurisdiction and invoice policy. The Dutch Tax and Customs Administration says to use consecutive invoice numbers in one or more series and that each invoice number may occur only once. That is Dutch guidance, not a universal statement of what every jurisdiction requires. See Belastingdienst guidance on invoice requirements.
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Which PostgreSQL numbering approach fits?
| Approach | Concurrent uniqueness | Gaps after rollback | Scope and contention | Failure handling |
|---|---|---|---|---|
MAX(number) + 1 without coordination |
Not safe by itself; concurrent transactions can choose the same value. A unique constraint can reject a collision. | Not a reliable gapless allocator; a rejected request does not resolve the race. | Can be scoped by tenant or series in the query, but that does not prevent concurrent collisions. | Application must handle unique-constraint errors; retrying may be necessary and is not a substitute for a sound allocation design. |
| PostgreSQL sequence | nextval allocates distinct values across concurrent sessions. |
Gaps are possible; values are not reclaimed after transaction aborts, among other documented behavior. | A sequence can be shared or separate sequences can be defined for different scopes; a shared sequence can become a common allocation point. | Usually no collision retry is needed for sequence allocation; the application still handles other insert failures. |
| Counter row updated in the invoice transaction | Safe when each series uses one counter row and the increment and invoice insert commit together. | Rollback undoes both the counter update and invoice insert, allowing the number to be reused. | Requests for the same series serialize on its row; separate tenants or series can update separate rows. | Handle transaction errors normally; keep the lock-holding transaction short. |
MAX(number) + 1 at SERIALIZABLE |
Serialization conflicts prevent unsafe concurrent outcomes if failures are handled correctly. | Can avoid gaps in the reported test when transactions succeed, but the approach incurs aborts and retries. | Conflicting work may serialize or abort; behavior depends on workload. | Application must retry serialization failures. In the cited test, some transactions still failed after the article’s retry limit. |
How do you number invoices per tenant in PostgreSQL?
For a gapless committed series, keep a counter row for each tenant and, if needed, each series. Increment that row and insert the invoice in the same transaction. The row lock coordinates requests for that series; the transaction ensures that a rollback undoes both changes.
Define the series and protect the business key
Decide whether numbering is per tenant, per tenant and year, or another series before writing the allocator. Represent that scope in the counter’s key and the invoice table’s uniqueness rule. For example, use (tenant_id, series) as the counter key and a unique constraint on (tenant_id, series, number) for invoices. If there is only one series per tenant, (tenant_id, number) may be the business key. The unique constraint remains an essential final integrity boundary even when allocation is correct.
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Allocate and insert in one short transaction
A PostgreSQL pattern is to use an upsert that increments the scoped counter and returns the allocated number, then insert the invoice before committing:
BEGIN;
INSERT INTO invoice_counters (tenant_id, series, last_number)
VALUES (42, '2026', 1)
ON CONFLICT (tenant_id, series)
DO UPDATE SET last_number = invoice_counters.last_number + 1
RETURNING last_number;
-- Use the returned value in the invoice insert.
INSERT INTO invoices (tenant_id, series, number, status)
VALUES (42, '2026', :returned_number, 'issued');
COMMIT;The first successful insert for a counter key starts at 1 in this example; a conflicting insert increments the existing counter by one. The returned value must be passed to the invoice insert within the same transaction. If the invoice insert fails or the transaction rolls back, the counter update rolls back too. Keep unrelated work—such as slow document generation—outside the transaction so the counter-row lock is held for as little time as practical.
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Leave drafts unnumbered until issuance
If draft invoices do not need official numbers, create them without one and allocate the number only when finalizing or issuing the invoice. This avoids holding a series number while a draft is edited or while other slow work occurs. Define separately how voided invoices, canceled issuance attempts, and failed issuance are recorded; the database allocation pattern alone does not set those accounting policies.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When is a sequence the better choice?
Use a PostgreSQL sequence when the requirement is distinct identifiers and occasional gaps are acceptable. PostgreSQL documents that nextval is atomic across concurrent sessions, so concurrent callers receive distinct values. It also explicitly warns: “PostgreSQL sequence objects cannot be used to obtain ‘gapless’ sequences.” Allocated values are not reclaimed after an abort, because doing so could block concurrent transactions. See the PostgreSQL 17 Sequence Manipulation Functions documentation.
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A sequence is therefore a good fit for surrogate keys or invoice identifiers where uniqueness matters more than a consecutive issued series. It is not a substitute for the counter-row transaction when policy requires gaplessness.
What does the performance evidence show?
Now-Next’s reported figures come from eight concurrent sessions running for 15 seconds with pgbench on PostgreSQL 17.10, on one machine with default settings. They are workload-specific measurements, not universal throughput guarantees:
| Method and test condition | Reported throughput | Reported outcome |
|---|---|---|
| One sequence, with 10% rollbacks | 12,131 invoices per second | 17,973 of 181,937 values skipped. |
MAX(number) + 1, default isolation |
10,769 invoices per second | 140,977 of 161,479 rows reused a number already issued in the test. |
MAX(number) + 1 under SERIALIZABLE, up to 20 retries |
1,475 invoices per second | No reported duplicates or gaps, but 26.6% of transactions failed. |
| Counter row, one tenant | 2,143 invoices per second | No reported duplicates or gaps in this test case. |
| Counter rows, requests spread over 1,000 tenants | 10,787 invoices per second | No reported duplicates or gaps in this test case. |
| Counter row with 10 ms of other work after taking the number | 94 invoices per second | The additional work extended the time the counter was held. |
| Counter row with 10 ms of other work before taking the number | 746 invoices per second | Moving the other work before allocation raised reported throughput in this added test. |
The contrast between one tenant and 1,000 tenants illustrates the contention trade-off in that workload: requests for one series share a counter row, while different tenants can update different rows. The benchmark did not test beyond eight sessions or examine crashes and replication, so it cannot establish how these designs behave under those conditions. Full benchmark details are in the Now-Next article.
How can you audit existing invoice numbers?
Run checks against the real numbering scope. If numbering is per tenant and series, partition or group by both; if the expected first number is 1, check that separately because a gap-between-rows query cannot detect a missing first number.
Find repeated numbers
SELECT tenant_id, series, number, COUNT(*) AS copies
FROM invoices
GROUP BY tenant_id, series, number
HAVING COUNT(*) > 1;Find gaps between existing numbers
WITH ordered AS (
SELECT tenant_id,
series,
number,
LEAD(number) OVER (
PARTITION BY tenant_id, series
ORDER BY number
) AS next_number
FROM invoices
)
SELECT tenant_id, series, number + 1 AS first_missing,
next_number - 1 AS last_missing
FROM ordered
WHERE next_number > number + 1;This identifies ranges missing between extant numbers. It does not determine whether a missing number is an error: that depends on rules for voided invoices, canceled documents, series boundaries, and failed issuance. Validate the first expected number and interpret findings against the policy for each series.
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