opt: alias @primary to PRIMARY KEY

[otan]

Release note (sql change): PRIMARY KEYs have been renamed to conform
to postgres (e.g. @tbl_col1_col2_pkey) in this release. To protect
certain use cases of backwards compatability, we also allow @primary
index hints to alias to the PRIMARY KEY, but only if no other index is
named primary.

[cockroach-teamcity]

This change is 

[RaduBerinde]

with some nits. Thanks!

[nit] "compatibility" in the release note

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

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pkg/sql/logictest/testdata/logic_test/table, line 862 at r1 (raw file):

  id INT PRIMARY KEY,
  b INT
); CREATE TABLE tbl_with_primary_named_index (

[nit] this is hard to read, just use two statement ok

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pkg/sql/logictest/testdata/logic_test/table, line 883 at r1 (raw file):

query T
EXPLAIN SELECT b FROM tbl_with_primary_named_index@primary

[nit] we try to keep EXPLAIN out of logictests, maybe this should be an execbuilder test?

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pkg/sql/logictest/testdata/logic_test/table, line 890 at r1 (raw file):

• scan
  missing stats
  table: tbl_with_primary_named_index@primary

This is not conclusive evidence that we're using the secondary index. Maybe add ORDER BY b or SELECT * (to force an index join)

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pkg/sql/opt/cat/utils.go, line 78 at r1 (raw file):

		}
		// Fallback to referencing @primary as the PRIMARY KEY.
		// Note indexes with "primary" as their name takes precedence above.

[nit] note that or "Note: "

[otan]

bors r=raduberinde

[craig]

Build failed:

• GitHub CI (Cockroach)

[otan]

bors r=raduberinde

[craig]

Build succeeded:

• GitHub CI (Cockroach)