Results of re-partition queries are affected by concurrent INSERT/DELETE/UPDATEs

While a re-partition query runs, especially a long one, we still continue to run INSERT/UPDATE/DELETE queries in parallel. Therefore, some tasks see the shards before the INSERT/UPDATE/DELETE queries, and other tasks see the shards after INSERT/UPDATE/DELETE queries.

How to replicate;

Load data:

``` sql
CREATE TABLE lineitem
(
l_orderkey BIGINT not null,
l_partkey INTEGER,
l_suppkey INTEGER,
l_linenumber INTEGER,
l_quantity DOUBLE PRECISION,
l_extendedprice DOUBLE PRECISION,
l_discount DOUBLE PRECISION,
l_tax DOUBLE PRECISION,
l_returnflag CHAR(1),
l_linestatus CHAR(1),
l_shipdate DATE,
l_commitdate DATE,
l_receiptdate DATE,
l_shipinstruct CHAR(25),
l_shipmode CHAR(10),
l_comment VARCHAR(44)
);

SELECT master_create_distributed_table('lineitem', 'l_orderkey', 'hash');
SELECT master_create_worker_shards('lineitem', 512, 1);

COPY lineitem from '/home/ubuntu/tpch_2_13_0/lineitem.tbl' WITH DELIMITER '|';
```

Set task tracker:

``` sql
set citus.task_executor_type to 'task-tracker';
```

Run a long running query

``` sql
SELECT
l_returnflag,
l_linestatus,
sum(l_quantity) as sum_qty,
sum(l_extendedprice) as sum_base_price,
sum(l_extendedprice * (1 - l_discount)) as sum_disc_price,
sum(l_extendedprice * (1 - l_discount) * (1 + l_tax)) as sum_charge,
avg(l_quantity) as avg_qty,
avg(l_extendedprice) as avg_price,
avg(l_discount) as avg_disc,
count(*) as count_order
FROM
lineitem
WHERE
l_shipdate <= date '1998-12-01' - interval '90' day
GROUP BY
l_returnflag,
l_linestatus
ORDER BY
l_returnflag,
l_linestatus;
```

Run the same query above again, but this time after starting the query, start to delete some rows concurrently.

``` bash
for i in `seq 1 1000`;
do
/usr/local/pgsql/bin/psql -d postgres -c "DELETE FROM lineitem WHERE l_orderkey = $i;"
done
```

Finally, observe that the result of the query above changed by the concurrent delete commands which are run after the original select query starts.