PostgreSQL has highly efficient and sophisticated transaction machinery that is capable of handling locks in a really fine-grained and efficient way. A few years ago, people came up with the idea of using this code to synchronize applications with each other. Thus, advisory locks were born.
When using advisory locks, it is important to mention that they won't go away on COMMIT as normal locks do. Therefore, it is really important to make sure that unlocking is done properly and in a totally reliable way.
If you decide to use an advisory lock, what you really lock is a number. So, this isn't about rows or data; it is really just a number. Here's how it works:
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Session 1 |
Session 2 |
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BEGIN; |
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SELECT pg_advisory_lock(15); |
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SELECT pg_advisory_lock(15); |
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It has to wait |
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COMMIT; |
It still has to wait |
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SELECT pg_advisory_unlock(15); |
It is still waiting |
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Lock is taken |
The first transaction will lock 15. The second transaction...