UPSERT, INSERT OR UPDATE, etc.
I need a way to UPDATE various columns of a row identified by a unique index, or INSERT a new row if it doesn't yet exist. That is, given
```
CREATE TABLE objects (
namespace string NOT NULL,
identifier string NOT NULL,
created_at time,
last_accessed_at time,
deleted_at time,
);
CREATE UNIQUE INDEX idx_objects_key ON objects (namespace, identifier);
```
I'd like to be able to express things like
```
BEGIN TRANSACTION;
INSERT INTO objects IF NOT EXISTS
(namespace, identifier, created_at) VALUES ($1, $2, $3);
UPDATE objects SET last_accessed_at = $3 WHERE namespace == $1 && identifier == $2;
COMMIT;
```
and
```
BEGIN TRANSACTION;
INSERT OR UPDATE objects
(namespace, identifier, last_accessed_at) VALUES ($1, $2, $3);
COMMIT;
```
issue