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Use a PostgreSQL range column and an exclusion constraint to reject reservations that overlap for the same slot. The database then enforces the rule when rows are written, even if two requests both appeared available to an earlier application check.
What rule should the database enforce?
A reservation is allowed unless another row has both the same slot identifier and an overlapping time range. In PostgreSQL, express those two comparisons as slot_id WITH = and reserved_during WITH && in a GiST exclusion constraint. PostgreSQL’s range documentation demonstrates this same equality-plus-overlap pattern for room reservations.
Use tstzrange when reservation endpoints represent absolute instants. PostgreSQL also provides tsrange for timestamps without time zones. With users in different time zones, normalize input to instants; if the product must preserve a particular local wall-clock interpretation, store that timezone separately as part of the application’s data model.
Build the reservation table and constraint
For a new table, the essential SQL is:
CREATE EXTENSION IF NOT EXISTS btree_gist;
CREATE TABLE parking_reservation (
reservation_id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
slot_id bigint NOT NULL,
reserved_during tstzrange NOT NULL,
EXCLUDE USING gist (
slot_id WITH =,
reserved_during WITH &&
)
);The btree_gist extension supplies GiST operator classes that provide B-tree-equivalent comparison behavior for many scalar types, including integer types. It lets the GiST constraint combine equality on slot_id with overlap on the range. It is not a replacement for a regular B-tree index when the only need is ordinary scalar lookup. See the PostgreSQL 15 btree_gist documentation.
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The extension is documented as trusted: a non-superuser with CREATE privilege on the current database can install it. A managed database provider may impose additional extension-availability rules, so confirm that prerequisite for your target service.
For an existing table, add the constraint after ensuring the extension is installed:
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ALTER TABLE parking_reservation
ADD CONSTRAINT parking_reservation_no_slot_overlap
EXCLUDE USING gist (
slot_id WITH =,
reserved_during WITH &&
);Both columns are NOT NULL here so every reservation has a definite slot and interval. Exclusion constraints use operator comparisons; if a comparison returns null, the row pair does not violate the exclusion rule. Choose nullability deliberately rather than relying on that behavior to represent a booking.
Choose range bounds so adjacent bookings work
Use half-open ranges, written [): the start is included and the end is excluded. For example:
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INSERT INTO parking_reservation (slot_id, reserved_during)
VALUES (42, tstzrange('2026-10-07 09:00+00', '2026-10-07 10:00+00', '[)'));A second reservation for slot 42 from 10:00 to 11:00 does not overlap the first, because the first interval excludes 10:00. A reservation that overlaps any part of the first interval for slot 42 is rejected. The same interval for another slot is permitted. PostgreSQL documents inclusive and exclusive range bounds in its range types reference.
Decide whether empty ranges have meaning in your domain and reject them if they do not. PostgreSQL 18’s WITHOUT OVERLAPS form disallows empty ranges and multiranges; check the behavior and syntax for the PostgreSQL version you deploy.
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What happens when two requests race?
An availability query can help the interface show open slots, but it is only a preview: concurrent requests can both observe an apparently free interval before either writes. The exclusion constraint is the final integrity check. PostgreSQL rejects a conflicting insert with an exclusion-constraint violation, as shown in its documented range example.
Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsHandle that violation as an ordinary booking conflict: tell the user the slot is no longer available and let them choose another slot or time. Do not treat an earlier availability result as a guarantee that a later insert will succeed.
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Which approach fits your PostgreSQL version?
| Approach | Best fit | Trade-off |
|---|---|---|
EXCLUDE USING GIST (slot_id WITH =, reserved_during WITH &&) |
Explicit operator-based rule on PostgreSQL versions supporting range exclusion constraints | Requires understanding the operator pair and commonly btree_gist for scalar equality |
UNIQUE (slot_id, reserved_during WITHOUT OVERLAPS) |
PostgreSQL 18 and later, where concise temporal-key syntax is available | Version-specific; the final key must be a range or multirange, and scalar GiST equality support may still require btree_gist |
| Application availability query alone | User-facing preview of open slots | Does not enforce the final rule when competing requests can both see the slot as available |
CHECK constraint that queries other rows |
Not suitable for this cross-row overlap rule | PostgreSQL warns that check constraints cannot safely enforce conditions involving other rows |
PostgreSQL 18 documents WITHOUT OVERLAPS as equivalent to GiST exclusion behavior for the non-overlapping range key. See PostgreSQL 18 CREATE TABLE. The explicit exclusion form makes the operators visible and is useful when supporting earlier versions.
Quick Recap
Check the resource model before deploying
- Slot identity:
slot_idmust identify the actual unit that cannot be booked twice. If a location has capacity greater than one, model distinct units or use a capacity-allocation design rather than treating one shared identifier as a single slot. - Cancellations: If canceled reservations should stop blocking availability, the constraint needs to account for status, for example through a conditional constraint. Verify the exact syntax and behavior for your PostgreSQL version before adopting that design.
- Deployment: Extension permissions, existing conflicting rows, migration locks, indexes, and partitioning can affect rollout. Validate these against your database and provider; the SQL above is the core rule, not a production migration plan.
- Performance: PostgreSQL creates an index for an exclusion constraint, and
btree_gistenables the mixed scalar-and-range comparison. No general parking-workload performance figure follows from that behavior; measure your own workload if performance is a concern.
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