DESCRIPTION: Fix PG upgrades when invalid rebalance strategies exist
Without this change an upgrade of a cluster with an invalid rebalance
strategy would fail with an error like this:
```
cache lookup failed for shard_cost_function with oid 6077337
CONTEXT: SQL statement "SELECT citus_validate_rebalance_strategy_functions(
NEW.shard_cost_function,
NEW.node_capacity_function,
NEW.shard_allowed_on_node_function)"
PL/pgSQL function citus_internal.pg_dist_rebalance_strategy_trigger_func() line 5 at PERFORM
SQL statement "INSERT INTO pg_catalog.pg_dist_rebalance_strategy SELECT
name,
default_strategy,
shard_cost_function::regprocedure::regproc,
node_capacity_function::regprocedure::regproc,
shard_allowed_on_node_function::regprocedure::regproc,
default_threshold,
minimum_threshold,
improvement_threshold
FROM public.pg_dist_rebalance_strategy"
PL/pgSQL function citus_finish_pg_upgrade() line 115 at SQL statement
```
This fixes that by disabling the trigger and simply re-inserting the
invalid rebalance strategy without checking. We could also silently
remove it, but this seems nicer.
Every move in the rebalancer algorithm results in an improvement in the
balance. However, even if the improvement in the balance was very small
the move was still chosen. This is especially problematic if the shard
itself is very big and the move will take a long time.
This changes the rebalancer algorithm to take the relative size of the
balance improvement into account when choosing moves. By default a move
will not be chosen if it improves the balance by less than half of the
size of the shard. An extra argument is added to the rebalancer
functions so that the user can decide to lower the default threshold if
the ignored move is wanted anyway.
Jelte Fennema-Nio
Jelte Fennema