Use Postgres CTE to update iteration numbers (fabric8-services#2319)

Use Postgres CTE to update iteration numbers

When explaining the updae before it looks like this:

```
postgres@172:postgres> explain UPDATE iterations iter SET number = seq.row_number
                       FROM (
                           SELECT id, space_id, created_at, row_number() OVER (PARTITION BY space_id ORDER BY created_at ASC)
                           FROM iterations
                       ) AS seq
                       WHERE iter.space_id = seq.space_id AND iter.id = seq.id;
+-----------------------------------------------------------------------------------------+
| QUERY PLAN                                                                              |
|-----------------------------------------------------------------------------------------|
| Update on iterations iter  (cost=2.06..2.16 rows=1 width=263)                           |
|   ->  Merge Join  (cost=2.06..2.16 rows=1 width=263)                                    |
|         Merge Cond: (seq.space_id = iter.space_id)                                      |
|         Join Filter: (iter.id = seq.id)                                                 |
|         ->  Subquery Scan on seq  (cost=1.03..1.09 rows=2 width=112)                    |
|               ->  WindowAgg  (cost=1.03..1.07 rows=2 width=48)                          |
|                     ->  Sort  (cost=1.03..1.03 rows=2 width=40)                         |
|                           Sort Key: iterations.space_id, iterations.created_at          |
|                           ->  Seq Scan on iterations  (cost=0.00..1.02 rows=2 width=40) |
|         ->  Sort  (cost=1.03..1.03 rows=2 width=187)                                    |
|               Sort Key: iter.space_id                                                   |
|               ->  Seq Scan on iterations iter  (cost=0.00..1.02 rows=2 width=187)       |
+-----------------------------------------------------------------------------------------+
```

Now the update looks better (notice absence of nested seq scans):

```
postgres@172:postgres> explain WITH iteration_numbers AS (
                           SELECT *, ROW_NUMBER() OVER(PARTITION BY space_id ORDER BY created_at ASC) AS num
                           FROM iterations
                       )
                       UPDATE iterations SET number = (SELECT num FROM iteration_numbers WHERE iteration_numbers.id = iterations.id);
+---------------------------------------------------------------------------------------------+
| QUERY PLAN                                                                                  |
|---------------------------------------------------------------------------------------------|
| Update on iterations  (cost=1.07..2.18 rows=2 width=191)                                    |
|   CTE iteration_numbers                                                                     |
|     ->  WindowAgg  (cost=1.03..1.07 rows=2 width=193)                                       |
|           ->  Sort  (cost=1.03..1.03 rows=2 width=185)                                      |
|                 Sort Key: iterations_1.space_id, iterations_1.created_at                    |
|                 ->  Seq Scan on iterations iterations_1  (cost=0.00..1.02 rows=2 width=185) |
|   ->  Seq Scan on iterations  (cost=0.00..1.11 rows=2 width=191)                            |
|         SubPlan 2                                                                           |
|           ->  CTE Scan on iteration_numbers  (cost=0.00..0.04 rows=1 width=8)               |
|                 Filter: (id = iterations.id)                                                |
+---------------------------------------------------------------------------------------------+
```

migration/sql-files/110-update-number-for-existing-iterations.sql

@@ -1,11 +1,10 @@
--- Assign a number to every existing iteration partitioned by their space and in
--- ascending creation order.
-UPDATE iterations SET number = seq.row_number
-FROM (
-    SELECT id, space_id, created_at, row_number() OVER (PARTITION BY space_id ORDER BY created_at ASC)
+--- Assign a number to every existing iteration partitioned by their space and in
+--- ascending creation order.
+WITH iteration_numbers AS (
+    SELECT *, ROW_NUMBER() OVER(PARTITION BY space_id ORDER BY created_at ASC) AS num
     FROM iterations
-) AS seq
-WHERE iterations.id = seq.id;
+)
+UPDATE iterations SET number = (SELECT num FROM iteration_numbers WHERE iteration_numbers.id = iterations.id);
 
 -- Make "number" a required column and add an index for faster querying over
 -- "space_id" and "number".
@@ -17,4 +16,4 @@ INSERT INTO number_sequences (space_id, table_name, current_val)
     SELECT space_id, 'iterations' "table_name", MAX(number)
     FROM iterations
     WHERE number IS NOT NULL
-    GROUP BY 1,2;
+    GROUP BY 1,2;