opt: adjust cost of scan with unbounded cardinality to avoid bad plans

[rytaft]

This commit tweaks the application of the unbounded cardinality penalty
in the coster to add it directly to the cost of scans and zigzag joins
rather than to the row count. This helps prevent an issue in which the
number of index columns could dominate the scan cost and result in
suboptimal plans.

Fixes #68556

Release note (bug fix): Fixed a regression in the optimizer's cost model
that could cause it to choose suboptimal plans when choosing between two
non-unique index scans with different numbers of columns per index.

[cockroach-teamcity]

This change is 

[mgartner]

Nice!

Reviewed 40 of 40 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @michae2 and @rytaft)

---

pkg/sql/opt/xform/coster.go, line 663 at r1 (raw file):

	// number of index columns does not have an outsized effect on the cost of
	// the scan. See issue #68556.
	baseCost += memo.Cost(c.largeCardinalityCostPenalty(scan.Relational().Cardinality, rowCount))

nit: if c.LargeCardinalityCostPenalty() returns a memo.Cost instead of a float64, this line is simplified.

---

pkg/sql/opt/xform/coster.go, line 1069 at r1 (raw file):

	// the number of index columns does not have an outsized effect on the cost of
	// the zigzag join. See issue #68556.
	cost += memo.Cost(c.largeCardinalityCostPenalty(join.Relational().Cardinality, rowCount))

ditto

[rytaft]

TFTR!

Reviewable status: complete! 0 of 0 LGTMs obtained (and 1 stale) (waiting on @mgartner and @michae2)

---

pkg/sql/opt/xform/coster.go, line 663 at r1 (raw file):

Previously, mgartner (Marcus Gartner) wrote…

nit: if c.LargeCardinalityCostPenalty() returns a memo.Cost instead of a float64, this line is simplified.

Done.

---

pkg/sql/opt/xform/coster.go, line 1069 at r1 (raw file):

Previously, mgartner (Marcus Gartner) wrote…

ditto

Done.

[mgartner]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (and 1 stale) (waiting on @michae2)

[rytaft]

bors r+

[craig]

Build failed (retrying...):

• GitHub CI (Cockroach)

[craig]

Build failed (retrying...):

• GitHub CI (Cockroach)

[craig]

Build failed:

• GitHub CI (Cockroach)

[rytaft]

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

---

error creating merge commit from 9a3d1ab to blathers/backport-release-21.1-68676: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.1.x failed. See errors above.

---

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is otan.}