Use existing number sequences instead of looking them up again (#2299)

In order to avoid a sequential table scan on the `work_items` DB table we take the already calculated values for the new `number_sequences` table from the old `work_item_number_sequences`.

Before this was the query plan for `INSERT` into the new `number_sequences` table:

```
EXPLAIN
SELECT space_id, 'work_items' "table_name", MAX(number) 
FROM work_items 
WHERE number IS NOT NULL 
GROUP BY 1,2;
+--------------------------------------------------------------------------------+
| QUERY PLAN                                                                     |
|--------------------------------------------------------------------------------|
| GroupAggregate  (cost=37097.49..38835.71 rows=37629 width=52)                  |
|   Group Key: space_id, 'work_items'::text                                      |
|   ->  Sort  (cost=37097.49..37437.97 rows=136193 width=52)                     |
|         Sort Key: space_id                                                     |
|         ->  Seq Scan on work_items  (cost=0.00..20824.93 rows=136193 width=52) |
|               Filter: (number IS NOT NULL)                                     |
+--------------------------------------------------------------------------------+
```

and now it is:

```
EXPLAIN
SELECT space_id, 'work_items' "table_name", current_val 
FROM work_item_number_sequences
GROUP BY 1,2;
+--------------------------------------------------------------------------------------------------------------------------------+
| QUERY PLAN                                                                                                                     |
|--------------------------------------------------------------------------------------------------------------------------------|
| Group  (cost=0.29..3541.66 rows=43872 width=52)                                                                                |
|   Group Key: space_id, 'work_items'::text                                                                                      |
|   ->  Index Scan using work_item_number_sequences_pkey on work_item_number_sequences  (cost=0.29..3322.30 rows=43872 width=52) |
+--------------------------------------------------------------------------------------------------------------------------------+
```

Thanks go out to @jarifibrahim for bringing my attention to this sequential table scan.

See #2291

migration/sql-files/106-number-sequences.sql

@@ -6,9 +6,6 @@ CREATE TABLE number_sequences (
     PRIMARY KEY (space_id, table_name)
 );
 
--- Delete old number table
-DROP TABLE work_item_number_sequences;
-
 -- Update existing iterations and areas with new "number" column and fill in the
 -- numbers in the order iterations and areas have been created.
 ALTER TABLE iterations ADD COLUMN number INTEGER;
@@ -32,11 +29,13 @@ ALTER TABLE areas ADD CONSTRAINT areas_space_id_number_idx UNIQUE (space_id, num
 -- Update the number_sequences table with the maximum for each table type
 
 INSERT INTO number_sequences (space_id, table_name, current_val)
-    SELECT space_id, 'work_items' "table_name", MAX(number) 
-    FROM work_items 
-    WHERE number IS NOT NULL 
+    SELECT space_id, 'work_items' "table_name", current_val 
+    FROM work_item_number_sequences
     GROUP BY 1,2;
 
+-- Delete old number table
+DROP TABLE work_item_number_sequences;
+
 INSERT INTO number_sequences (space_id, table_name, current_val)
     SELECT space_id, 'iterations' "table_name", MAX(number)
     FROM iterations

migration/sql-test-files/106-number-sequences.sql

@@ -39,6 +39,11 @@ VALUES
     (current_setting('wi3.id')::uuid, current_setting('wit.id')::uuid, current_setting('sp2.id')::uuid, '{"system.title":"WI3"}'::json, 1, '2018-09-17 12:01'),
     (current_setting('wi4.id')::uuid, current_setting('wit.id')::uuid, current_setting('sp2.id')::uuid, '{"system.title":"WI4"}'::json, 2, '2018-09-17 17:01');
 
+INSERT INTO work_item_number_sequences (space_id, current_val)
+VALUES 
+    (current_setting('sp1.id')::uuid, 2),
+    (current_setting('sp2.id')::uuid, 2);
+
 INSERT INTO iterations (id, name, path, space_id, created_at) 
 VALUES 
     (current_setting('iter1.id')::uuid, 'iteration 1', replace(current_setting('iter1.id'), '-', '_')::ltree, current_setting('sp1.id')::uuid, '2018-09-17 16:01'),