Transaction completion timing

Say we have two transactions run sequentially: T1 writes some data, and T2 reads the written data. There is a non-zero time delay between the apparent T1 commit, and the subsequent T2 query.

Is there any guarantee that the data written in T1 will be visible to the query in T2?

We have a situation in our system where it would appear that the answer is NO.

Based on my limited understanding of PostgreSQL internals, although T1 has apparently been committed (from the client’s perspective), that doesn’t necessarily mean that it has been completed inside the database. It seems that a commit only means that the transaction has been added to the WAL, but it doesn’t mean the transaction results are available to subsequent transactions. At some later date, the database completes T1, and then makes it visible to future transactions. Only after that point would the data be visible to T2. So if T2 runs too soon after T1, T1’s writes won’t be visible to T1.

Can someone clarify this for me?

Thanks so much,
Steve
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