Fix performance regression in previous_version function (#366)

#365 changed the definition of
`previous_version` and introduced a performance regression.

Update the function with extra `WHERE` clauses so that the recursive CTE
can take better advantage of the table PK index.

On a large (~1M record) `pgroll.migrations` table the query analysis for
the recursive CTE is:

**Before**:

```
+----------------------------------------------------------------------------------------------------------------------------------------------------------+
| QUERY PLAN                                                                                                                                               |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| CTE Scan on ancestors  (cost=1819977.60..1819978.62 rows=51 width=214) (actual time=1163.608..6463.980 rows=1 loops=1)                                   |
|   CTE ancestors                                                                                                                                          |
|     ->  Recursive Union  (cost=0.00..1819977.60 rows=51 width=114) (actual time=1163.606..6463.977 rows=1 loops=1)                                       |
|           ->  Seq Scan on migrations  (cost=0.00..442329.71 rows=1 width=114) (actual time=1163.604..6385.489 rows=1 loops=1)                            |
|                 Filter: (name = latest_version('bb_00bo06t68d5ot7i5k5m8um70kg_bb19qc'::name))                                                            |
|                 Rows Removed by Filter: 1193140                                                                                                          |
|           ->  Nested Loop  (cost=0.55..137764.74 rows=5 width=114) (actual time=78.485..78.486 rows=0 loops=1)                                           |
|                 ->  WorkTable Scan on ancestors a  (cost=0.00..0.20 rows=10 width=36) (actual time=0.001..0.002 rows=1 loops=1)                          |
|                 ->  Index Scan using migrations_pkey on migrations m  (cost=0.55..13776.44 rows=1 width=110) (actual time=78.459..78.459 rows=0 loops=1) |
|                       Index Cond: (name = a.parent)                                                                                                      |
|                       Filter: ((migration_type)::text = 'inferred'::text)                                                                                |
|                       Rows Removed by Filter: 1                                                                                                          |
| Planning Time: 0.764 ms                                                                                                                                  |
| JIT:                                                                                                                                                     |
|   Functions: 11                                                                                                                                          |
|   Options: Inlining true, Optimization true, Expressions true, Deforming true                                                                            |
|   Timing: Generation 2.222 ms, Inlining 29.422 ms, Optimization 51.028 ms, Emission 24.552 ms, Total 107.225 ms                                          |
| Execution Time: 6466.347 ms                                                                                                                              |
+----------------------------------------------------------------------------------------------------------------------------------------------------------+
```

**After**:

```
+----------------------------------------------------------------------------------------------------------------------------------------------------+
| QUERY PLAN                                                                                                                                         |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| CTE Scan on ancestors  (cost=987.99..988.21 rows=11 width=214) (actual time=0.277..0.379 rows=1 loops=1)                                           |
|   CTE ancestors                                                                                                                                    |
|     ->  Recursive Union  (cost=0.55..987.99 rows=11 width=114) (actual time=0.275..0.377 rows=1 loops=1)                                           |
|           ->  Index Scan using history_is_linear on migrations  (cost=0.55..128.36 rows=1 width=114) (actual time=0.274..0.354 rows=1 loops=1)     |
|                 Index Cond: (schema = 'bb_00bo06t68d5ot7i5k5m8um70kg_bb19qc'::name)                                                                |
|                 Filter: (name = latest_version('bb_00bo06t68d5ot7i5k5m8um70kg_bb19qc'::name))                                                      |
|                 Rows Removed by Filter: 5                                                                                                          |
|           ->  Nested Loop  (cost=0.55..85.95 rows=1 width=114) (actual time=0.020..0.020 rows=0 loops=1)                                           |
|                 ->  WorkTable Scan on ancestors a  (cost=0.00..0.20 rows=10 width=100) (actual time=0.001..0.001 rows=1 loops=1)                   |
|                 ->  Index Scan using migrations_pkey on migrations m  (cost=0.55..8.57 rows=1 width=110) (actual time=0.017..0.017 rows=0 loops=1) |
|                       Index Cond: ((schema = a.schema) AND (name = a.parent))                                                                      |
|                       Filter: ((migration_type)::text = 'inferred'::text)                                                                          |
|                       Rows Removed by Filter: 1                                                                                                    |
| Planning Time: 0.744 ms                                                                                                                            |
| Execution Time: 0.459 ms                                                                                                                           |
+----------------------------------------------------------------------------------------------------------------------------------------------------+
```

ie the recursive portion of the table is able to take advantage of the
table's PK (defined on `(schema, name)`).

pkg/state/state.go

@@ -75,14 +75,14 @@ STABLE;
 CREATE OR REPLACE FUNCTION %[1]s.previous_version(schemaname NAME) RETURNS text
 AS $$
   WITH RECURSIVE ancestors AS (
-  SELECT name, parent, migration_type, 0 AS depth FROM %[1]s.migrations
-    WHERE name = %[1]s.latest_version(schemaname)
+  SELECT name, schema, parent, migration_type, 0 AS depth FROM %[1]s.migrations
+    WHERE name = %[1]s.latest_version(schemaname) AND schema = schemaname
 
   UNION ALL
 
-  SELECT m.name, m.parent, m.migration_type, a.depth + 1
+  SELECT m.name, m.schema, m.parent, m.migration_type, a.depth + 1
     FROM %[1]s.migrations m
-    JOIN ancestors a ON m.name = a.parent
+    JOIN ancestors a ON m.name = a.parent AND m.schema = a.schema
   )
   SELECT a.name FROM ancestors a
     JOIN information_schema.schemata s