Query table_comments without unrolling with UNION ALL

`table_comments` is equally "hard" for the engine as `information_schema.columns`. Avoid optimizing the query on the `dbt` side by sending `UNION ALL` with individual schemas. Let the engine figure out the optimal strategy.
starburstdata · Oct 3, 2023 · 8029d67 · 8029d67
1 parent 6d4cc2a
commit 8029d67
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diff --git a/.changes/unreleased/Under the Hood-20231003-161428.yaml b/.changes/unreleased/Under the Hood-20231003-161428.yaml
@@ -0,0 +1,7 @@
+kind: Under the Hood
+body: Query table_comments without unrolling with UNION ALL
+time: 2023-10-03T16:14:28.238801+02:00
+custom:
+  Author: findepi
+  Issue: ""
+  PR: "357"
diff --git a/dbt/include/trino/macros/catalog.sql b/dbt/include/trino/macros/catalog.sql
@@ -33,12 +33,11 @@
             where
                 table_schema != 'information_schema'
                 and
-                table_schema in ('{{ schemas | join("','") | lower }}')
+                schema_name in ('{{ schemas | join("','") | lower }}')
 
         ),
 
         table_comment as (
-            {%- for schema in schemas %}
             select
                 catalog_name as "table_database",
                 schema_name as "table_schema",
@@ -51,11 +50,7 @@
                 and
                 schema_name != 'information_schema'
                 and
-                schema_name = '{{ schema | lower }}'
-            {% if not loop.last %}
-            union all
-            {% endif %}
-            {%- endfor %}
+                table_schema in ('{{ schemas | join("','") | lower }}')
         )
 
         select