planner: wrong Join estimation depending on uninitialized or missing stats

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Root Cause:
1. In join estimation, we use left-rows*right-rows/max(left-ndv, right-ndv).
2. When deriving NDV from index-stats, we didn't check whether it was loaded or missing. After adding a new index without analyzing, the index-stats might be missing,
3. If it's missing, we'll use the wrong NDV=0 or 1 to calculate join estimation, and finally get a high error result.

See the pic below, the join estimation has a huge error:

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

v7.5.6

[qw4990]

Join Estimation depends on NDV-Group, but when building NDV-Group from stats, we don't consider whether this stats is loaded...