sql: support expressions as ON CONFLICT targets

[mgartner]

We don't currently support expressions as ON CONFLICT targets. This means that unique expression indexes (still a WIP at the time this was written) cannot be selected as arbiters for INSERT .. ON CONFLICT statements. This will require adding support for expression ON CONFLICT targets to the parser and the AST, and updating optbuilder's arbiter logic to select arbiters based on the expression.

Example:

defaultdb> SET experimental_enable_expression_indexes=true;
SET

defaultdb> CREATE TABLE t (a INT, b INT, UNIQUE INDEX ((a + b)));
CREATE TABLE

defaultdb> INSERT INTO t VALUES (1, 2) ON CONFLICT ((a + b)) DO NOTHING;
invalid syntax: statement ignored: at or near "(": syntax error
SQLSTATE: 42601
DETAIL: source SQL:
INSERT INTO t VALUES (1, 2) ON CONFLICT ((a + b)) DO NOTHING
                                         ^
HINT: try \h INSERT

defaultdb> INSERT INTO t VALUES (1, 2) ON CONFLICT ((a + b)) DO UPDATE SET a = 10;
invalid syntax: statement ignored: at or near "(": syntax error
SQLSTATE: 42601
DETAIL: source SQL:
INSERT INTO t VALUES (1, 2) ON CONFLICT ((a + b)) DO UPDATE SET a = 10
                                         ^
HINT: try \h INSERT

Jira issue: CRDB-8765

[mgartner]

@ianjevans this will likely be a known limitation for expression indexes in the 21.2 release.

[ecwall]

These flags can be enabled after this is fixed: cockroachdb/activerecord-cockroachdb-adapter#250

[mgartner]

I don't think the linked PR fixes this issue. Reopening.

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it in
10 days to keep the issue queue tidy. Thank you for your contribution
to CockroachDB!