opt: support multiple arbiter indexes for INSERT ON CONFLICT DO UPDATE

[mgartner]

We currently do not support multiple arbiter indexes for INSERT ON CONFLICT DO UPDATE.

Postgres:

marcus=# CREATE TABLE t (a INT, b INT);
CREATE TABLE

marcus=# CREATE UNIQUE INDEX i ON t (a) WHERE b > 0;
CREATE INDEX

marcus=# CREATE UNIQUE INDEX i2 ON t (a) WHERE b < 0;
CREATE INDEX

marcus=# INSERT INTO t VALUES (1, 1) ON CONFLICT (a) WHERE b < 0 AND b > 0 DO UPDATE SET b = 0;
INSERT 0 1

CockroachDB:

root@127.0.0.1:63997/defaultdb> SET experimental_partial_indexes=on;
SET

root@127.0.0.1:63997/defaultdb> CREATE TABLE t (a INT, b INT);
CREATE TABLE

root@127.0.0.1:63997/defaultdb> CREATE UNIQUE INDEX i ON t (a) WHERE b > 0;
CREATE INDEX

root@127.0.0.1:63997/defaultdb> CREATE UNIQUE INDEX i2 ON t (a) WHERE b < 0;
CREATE INDEX

root@127.0.0.1:63997/defaultdb> INSERT INTO t VALUES (1, 1) ON CONFLICT (a) WHERE b < 0 AND b > 0 DO UPDATE SET b = 0;
ERROR: unimplemented: there are multiple unique or exclusion constraints matching the ON CONFLICT specification
SQLSTATE: 0A000
HINT: You have attempted to use a feature that is not yet implemented.
See: https://go.crdb.dev/issue/53170

Workaround

The current workarounds are to:

• Make the ON CONFLICT WHERE clause more strict so that it only matches a single unique index.
• Or, drop all but one unique partial index that matches the ON CONFLICT WHERE clause.

Notes

In order to do this, we'll have to loop through all arbiter indexes and left-join them together to build a composite canary column that dictates whether a row should be inserted or updated. This is similar to what is done for INSERT ON CONFLICT DO NOTHING with the added complexity of the canary column.

Jira issue: CRDB-3894