Statistics of TIMESTAMP column is stored as datetime string without timezone information

[winoros]

Bug Report

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1. Minimal reproduce step (Required)

As you can see, the type of this column is TIMESTAMP, but we store its histogram with datetime string without timezone information. And the timezone is not UTC, it's decided by the session executing the ANALYZE command.

This is not correct and will introduce an estimation error.

[winoros]

This is the root cause of #41985

[winoros]

We've already stored the histogram of TIMESTAMP to string value. So it's not easy to store it at its original value for compatibility issues.

A possible fix is that we always store the datetime string with UTC timezone, and do conversion when doing row count estimation.

[winoros]

TopN or index's histogram is correct. They're following the normal encoding&decoding procedure.

[winoros]

It's a long-existing issue. I think i would not solve it before 8.1.0 is released. I'll fix it in the minor version.