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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →A plain InnoDB SELECT reads data; it does not reserve the row against another transaction. If two purchases both read that the last unit is available, both can proceed unless the availability check and stock change are coordinated. Use SELECT ... FOR UPDATE inside a transaction when the application must inspect the current row before deciding whether to decrement stock.
Why a plain SELECT can oversell
Imagine an inventory row with stock = 1. Two purchase transactions run an ordinary SELECT and each sees one unit. Because these are nonlocking consistent reads, neither transaction prevents the other from changing the row after the read. Both application processes may therefore conclude that they can sell the item.
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The weakness is the gap between observing a value and acting on it: the read did not give either transaction exclusive control of that inventory row. MySQL’s 8.4 Reference Manual warns that when a transaction queries data and then inserts or updates related data, a regular SELECT does not provide enough protection.
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How SELECT … FOR UPDATE changes the flow
A locking read requests an exclusive lock on the rows it selects. Keep the read, availability check, and stock update in one transaction so a competing transaction that tries to lock or modify the same row must wait until the first transaction commits or rolls back.
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START TRANSACTION;
SELECT stock
FROM inventory
WHERE product_id = ?
FOR UPDATE;
-- In application code, verify stock >= requested_quantity.
-- If insufficient, ROLLBACK and report unavailable.
UPDATE inventory
SET stock = stock - ?
WHERE product_id = ?;
COMMIT;Run the availability check only after the locking read returns. If the product is missing, stock is insufficient, or the operation fails, roll back rather than committing a partial reservation. The example shows the transaction shape; production code should also reject nonpositive requested quantities, check affected rows, and use the database driver’s transaction API correctly.
Choose a query that locks the intended rows
FOR UPDATE does not guarantee that only the one row you have in mind will be locked, regardless of the predicate. The access path and index matter. With a unique-index equality lookup, InnoDB can generally lock the matching record without locking the preceding gap. Range scans, nonunique indexes, or a scan without a suitable index can create a wider footprint, including range or next-key locking where applicable.
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- For one inventory row, filter by its unique key where possible.
- Check the query plan against the actual schema to confirm that the intended index is used.
- Account for the deployed MySQL version and isolation level: exact lock behavior depends on these as well as the index and query plan.
See the MySQL manual’s InnoDB locks set by statements for details on how statements and access paths affect locking.
Understand the isolation-level interaction
InnoDB’s default isolation level is REPEATABLE READ. Ordinary consistent reads in a transaction use a snapshot established by the first consistent read, while locking reads use locking semantics. Mixing those read types casually can leave application logic reasoning from different views of the data. MySQL discusses this distinction in its transaction isolation-level documentation.
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For an inventory decision, use the locking read as the basis for the check and keep the dependent update in the same transaction. Confirm version-specific behavior in the manual for the MySQL release you deploy; the relevant documentation spans multiple manual versions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Account for waits and deadlocks
Locking protects the decision by limiting concurrent access, but it can also make transactions wait. Concurrent transactions may deadlock, so application code should handle transaction failures and retry where appropriate. Keep the locked section short: avoid unrelated slow work before commit, and when a transaction needs several inventory rows, access them in a consistent order where practical. MySQL describes these concurrency behaviors in its InnoDB deadlock documentation.
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When an atomic conditional update may fit better
If the application does not need to inspect row state before writing, a conditional update can express the reservation in one statement: decrement only when stock >= requested_quantity, then inspect the affected-row count to determine whether it succeeded. This avoids a separate application-level check, but it may not suit workflows that need to inspect additional state or coordinate several records.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsChoose between this approach and SELECT ... FOR UPDATE based on what the transaction must decide, which rows it must coordinate, and how the application detects insufficient stock and handles transaction failures. The MySQL locking documentation establishes the locking behavior, but does not benchmark these application-level designs.
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