[FEATURE]: BATCH API in node-postgres

[jakeleventhal]

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Reopening #2291

Saw this was closed, but wondering why. Assume the following code:

await db.transaction(async (tx) => {
  await tx.update(...);
  await tx.update(...);
});

This will perform 2 updates inside a transaction. If one fails, the transaction fails. We can speed this up by putting each update inside a Promise.all, but we are still making 3 trips to our database:

1. start transaction
2. update rows
3. end transaction

It would be much more efficient to do something like

db.batch([
  db.update(...),
  db.update(...)
]);

Under the hood this could produce a single SQL statement that does the following:

BEGIN;
UPDATE table1 SET column_name1 = 'new_value' WHERE condition1;
UPDATE table2 SET column_name2 = 'new_value' WHERE condition2;
COMMIT;

Now I only have one chunk of SQL being sent to the database rather than 3.