After altering a subscription, we should wait until the updated table
sync data has been fetched by the subscriber.
$node_subscriber->safe_psql('postgres',
"ALTER SUBSCRIPTION tap_sub REFRESH PUBLICATION");
+$node_subscriber->poll_query_until('postgres', $synced_query)
+ or die "Timed out while waiting for subscriber to synchronize data";
+
# Add replica identity column. (The serial is not necessary, but it's
# a convenient way to get a default on the new column so that rows
# from the publisher that don't have the column yet can be inserted.)