opt: add helper functions for partial indexes and scans

[mgartner]

This commit adds helper functions to determine if indexes are partial
indexes, if scans operate on partial indexes, and for retrieving partial
index predicate expressions.

Within the optimizer TableMeta.PartialIndexPredicates is now the
source of truth that should be used for answering these types of
questions. The catalog Index.Predicate function should only be used by
the optbuilder.

This commit also makes certain that there is an entry in the
TableMeta.PartialIndexPredicates map for every partial index on the
table, even if the partial index predicate is non-immutable. This makes
the map a safe source of truth for determining which indexes are
partial.

Release note: None

[cockroach-teamcity]

This change is 

[RaduBerinde]

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

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pkg/sql/opt/memo/expr.go, line 812 at r1 (raw file):

}

// IsPartialIndex returns true if the table's index at the given ordinal is

[nit] should these be methods of TableMeta?

[mgartner]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @RaduBerinde)

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pkg/sql/opt/memo/expr.go, line 812 at r1 (raw file):

Previously, RaduBerinde wrote…

[nit] should these be methods of TableMeta?

IsPartialIndex could be, but PartialIndexPredicate cannot because it returns a memo.FiltersExpr, and the memo package can't be used from the opt package. I wanted this function to do the type cast rather than have each caller do the cast. And I think it's best to keep both of these functions together rather than put one in TableMeta and one here.

[mgartner]

bors r=RaduBerinde

[RaduBerinde]

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pkg/sql/opt/memo/expr.go, line 812 at r1 (raw file):

Previously, mgartner (Marcus Gartner) wrote…

IsPartialIndex could be, but PartialIndexPredicate cannot because it returns a memo.FiltersExpr, and the memo package can't be used from the opt package. I wanted this function to do the type cast rather than have each caller do the cast. And I think it's best to keep both of these functions together rather than put one in TableMeta and one here.

👍

[craig]

Build succeeded

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