sql, opt: avoid full scans in mutation queries with cost flags

[rytaft]

opt: convert memo.Cost to a struct with CostFlags

This is a mechanical change that will set up the ability to use
CostFlags in a future commit.

Release note: None

opt,sql: support hint to avoid full scan

Added a hint to avoid full scans (see release note below for details).
To support this change, added a field to memo.Cost with a new type
memo.CostFlags, which contains a number of boolean flags and supports
"multi-dimensional costing". This allows the optimizer to compare plans
based on the flags set in addition to the single-dimensional float64
cost. For example, plans with the new FullScanPenalty cost flag enabled
will always be more expensive than plans without any cost flags, even
if the base float64 cost is lower.

The new CostFlags type also includes a flag for HugeCostPenalty, which
must be set for plans with hugeCost. This ensures that existing
hints that use hugeCost still work if some other cost flags are set,
since HugeCostPenalty takes precedence over other cost flags.

This new CostFlags field is needed to support hints that do not cause an
error if the optimizer cannot find a plan complying with the hint. This
is needed because the previous approach of simply using hugeCost to
avoid certain plans meant that if such plans were unavoidable, we could
not effectively compare plans with cost greater than hugeCost due to
loss of floating point precision.

Informs #79683

Release note (sql change): Added support for a new index hint,
AVOID_FULL_SCAN, which will prevent the optimizer from planning a
full scan for the specified table if any other plan is possible. The
hint can be used in the same way as other existing index hints. For
example, SELECT * FROM table_name@{AVOID_FULL_SCAN};. This hint is
similar to NO_FULL_SCAN, but will not error if a full scan cannot be
avoided. Note that normally a full scan of a partial index would not
be considered a "full scan" for the purposes of the NO_FULL_SCAN and
AVOID_FULL_SCAN hints, but if the user has explicitly forced the
partial index via FORCE_INDEX=index_name, we do consider it a full
scan.

sql,opt: add setting avoid_full_table_scans_in_mutations

Fixes #79683

Release note (sql change): Added a new session setting
avoid_full_table_scans_in_mutations, which when set to true, causes
the optimizer to avoid planning full table scans for mutation queries
if any other plan is possible. It now defaults to true.

opt: remove a stale comment above optbuilder.buildScan

Removed a comment that references a function parameter that no longer
exists.

Release note: None

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This change is 

[rytaft]

pkg/sql/opt/xform/testdata/rules/limit line 2770 at r4 (raw file):

                ├── scan t82730a@t82730a_col1_1_col1_3_key
                │    ├── columns: col1_0:7!null col1_1:8 col1_3:9
                │    ├── constraint: /8/9

I'm looking into this.

[rytaft]

pkg/sql/opt/xform/testdata/rules/limit line 2844 at r4 (raw file):

                ├── scan t82730b@t82730b_col1_1_col1_3_key [as=tab_41831]
                │    ├── columns: col1_0:7!null col1_1:8 col1_3:9
                │    ├── constraint: /8/9

Ditto

[rytaft]

Reviewable status: complete! 0 of 0 LGTMs obtained

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pkg/sql/opt/xform/testdata/rules/limit line 2770 at r4 (raw file):

Previously, rytaft (Rebecca Taft) wrote…

I'm looking into this.

This is an unrelated bug: #137994

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pkg/sql/opt/xform/testdata/rules/limit line 2844 at r4 (raw file):

Previously, rytaft (Rebecca Taft) wrote…

Ditto

See #137994

[rytaft]

pkg/sql/opt/memo/cost.go line 14 at r4 (raw file):

// TODO: Need more details about what one "unit" of cost means.
type Cost struct {
	Cost  float64

Happy to bikeshed on names here -- maybe the "Cost" data member should just be "C"?