explain: print "FULL SCAN (SOFT LIMIT)" for full scans with soft limits

[michae2]

Change EXPLAIN output of full scans with soft limits to
"FULL SCAN (SOFT LIMIT)" instead of "FULL SCAN". This will help
distinguish them from unlimited full scans.

So now we print three different messages for three kinds of full scans:

• For full scans with hard limits we print "LIMITED SCAN".
• For full scans with soft limits we print "FULL SCAN (SOFT LIMIT)".
• For unlimited full scans we print "FULL SCAN".

Release note (sql change): Change EXPLAIN output of full scans with
soft limits to "FULL SCAN (SOFT LIMIT)" instead of "FULL SCAN", to
distinguish them from unlimited full scans. Unlimited full scans always
scan the entire index. Full scans with soft limits could scan the entire
index, but usually halt early once enough rows have been found to
satisfy their parent operator.

[cockroach-teamcity]

This change is 

[rytaft]

LIMITED FULL SCAN sounds a bit like a contradiction, but I'm struggling to think of a better idea. cc @vy-ton and @kevin-v-ngo for awareness and in case you guys have an idea about how to make this more clear.

Reviewed 10 of 10 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @msirek)

[michae2]

LIMITED FULL SCAN sounds a bit like a contradiction, but I'm struggling to think of a better idea. cc @vy-ton and @kevin-v-ngo for awareness and in case you guys have an idea about how to make this more clear.

Yeah, I was also struggling to come up with something better. I also thought about POTENTIALLY-LIMITED FULL SCAN and SHORT-CIRCUITING FULL SCAN and FULL SCAN (SOFT LIMIT) but these all seemed too verbose. I'm open to any suggestions!

[mgartner]

Some more ideas to throw into the mix: HALTABLE FULL SCAN or HALTING FULL SCAN. I also thought of PREEMPTIBLE FULL SCAN but I think preemption would mean stopping the scan and restarting it later.

Reviewed 10 of 10 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @msirek)

[michae2]

The more I think about it, the more I dislike LIMITED FULL SCAN. How do people feel about POSSIBLE FULL SCAN?

[DrewKimball]

FULL SCAN (SOFT LIMIT) seems most intuitive to me. Another option might be SOFT LIMIT SCAN or SOFT LIMITED SCAN if we're ok with losing the FULL. PIPELINED FULL SCAN is another possibility, though maybe that one is too far removed from LIMIT.

How do people feel about POSSIBLE FULL SCAN?

POSSIBLE FULL SCAN and POTENTIAL FULL SCAN also sound good.

[mgartner]

I like FULL SCAN (SOFT LIMIT), though we'd have to educate users what "soft limit" means because I don't think it'd be immediately obvious to most.

[rytaft]

I also like FULL SCAN (SOFT LIMIT). That seems to me like the best of the options we've discussed. We'll need to make sure the docs are updated to explain it cc @taroface who will be taking over SQL Queries docs soon.

[michae2]

Thanks for the feedback, everyone!

bors r=rytaft,mgartner

[craig]

Build succeeded:

• GitHub CI (Cockroach)