Fix incremental delete+insert SQL

resolves dbt-labs#150

Problem
The delete query for the 'delete+insert' incremental_strategy with 2+ unique_key columns is VERY inefficient. In many cases, it will hang and never return for deleting small amounts of data (<100K rows).

Solution
Improve the query by switching to a much more efficient delete strategy:

```
delete from table1
where (col1, col2) in (
    select distinct col1, col2 from table1_tmp
)
```

.changes/unreleased/Fixes-20240410-163601.yaml

@@ -0,0 +1,6 @@
+kind: Fixes
+body: Fix incremental delete+insert SQL
+time: 2024-04-10T16:36:43.253228-05:00
+custom:
+ Author: ataft
+ Issue: "150"

dbt/include/global_project/macros/materializations/models/incremental/merge.sql

@@ -61,34 +61,23 @@
  {%- set dest_cols_csv = get_quoted_csv(dest_columns | map(attribute="name")) -%}
 
  {% if unique_key %}
- {% if unique_key is sequence and unique_key is not string %}
- delete from {{target }}
- using {{ source }}
- where (
- {% for key in unique_key %}
- {{ source }}.{{ key }} = {{ target }}.{{ key }}
- {{ "and " if not loop.last}}
- {% endfor %}
- {% if incremental_predicates %}
- {% for predicate in incremental_predicates %}
- and {{ predicate }}
- {% endfor %}
- {% endif %}
- );
- {% else %}
- delete from {{ target }}
- where (
- {{ unique_key }}) in (
- select ({{ unique_key }})
- from {{ source }}
- )
- {%- if incremental_predicates %}
- {% for predicate in incremental_predicates %}
- and {{ predicate }}
- {% endfor %}
- {%- endif -%};
-
+ {% if unique_key is string %}
+ {% set unique_key = [unique_key] %}
  {% endif %}
+
+ {%- set unique_key_str = unique_key|join(', ') -%}
+
+ delete from {{ target }}
+ where ({{ unique_key_str }}) in (
+ select distinct {{ unique_key_str }}
+ from {{ source }}
+ )
+ {%- if incremental_predicates %}
+ {% for predicate in incremental_predicates %}
+ and {{ predicate }}
+ {% endfor %}
+ {%- endif -%};
+
  {% endif %}
 
  insert into {{ target }} ({{ dest_cols_csv }})