KB show queries using MVs

knowledgebase/how_to_display_queries_using_MV.md

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+---
+date: 2023-03-24
+---
+
+# How to display queries using Materialized Views?
+
+**Question:** How do I show all queries involving materialized views in the last 60m?
+
+**Answer:**
+
+This query will display all queries directed towards Materialized Views considering that:
+
+- we can leverage the `create_table_query` field in `system.tables` table to identify what tables are explicit (`TO`) recipient of MVs;
+- we can track back (using `uuid` and the name convention `.inner_id.<uuid>`) what tables are implicit recipient of MVs;
+
+We can also configure how long back in time we want to look, by changing the value (`60` m by default) in the initial query CTE
+
+```sql
+WITH(60) -- default 60m
+AS timeRange,
+(
+    --prepare names of possible implicit MV hidden target tables for *any* table with NON NULL uuid
+    SELECT groupArray(
+            concat('default.`.inner_id.', toString(uuid), '`')
+        )
+    FROM clusterAllReplicas(default, system.tables)
+    WHERE notEmpty(uuid)
+) AS MV_implicit_possible_hidden_target_tables_names_array,
+(
+    --captures MV name and target tables (if TO is specified)
+    --TODO it seems that extract will return just the first capturing group :( replace with regexpExtract once available
+    SELECT arrayFilter(
+            x->x != '',
+            --remove empty captures
+            groupArray(
+                extract(
+                    create_table_query,
+                    '^CREATE MATERIALIZED VIEW\s(\w+\.\w+)\s(?:TO\s(\S+))?'
+                )
+            )
+        )
+    FROM clusterAllReplicas(default, system.tables)
+    WHERE engine = 'MaterializedView'
+) AS MV_explicit_target_tables_names_array
+SELECT event_time,
+    query,
+    tables as "MVs tables"
+FROM clusterAllReplicas(default, system.query_log)
+WHERE (
+        -- only SELECT within 60m
+        event_time > now() - toIntervalMinute(timeRange)
+        AND startsWith(query, 'SELECT')
+    ) -- check either that query involves implicit MV target table names
+    AND (
+        hasAny(
+            tables,
+            MV_implicit_possible_hidden_target_tables_names_array
+        )
+        OR -- check that query involves explicit MV target table
+        hasAny(tables, MV_explicit_target_tables_names_array)
+    )
+ORDER BY event_time DESC;
+```
+
+expected output:
+
+```sql
+| event_time          | query                                                                                          | MVs tables                                                            |
+| ------------------- | ---------------------------------------------------------------------------------------------- | --------------------------------------------------------------------- |
+| 2023-02-23 08:14:14 | SELECT     rand(),* FROM     default.sum_of_volumes,     default.big_changes,     system.users | ["default.big_changes_mv","default.sum_of_volumes_mv","system.users"] |
+| 2023-02-23 08:04:47 | SELECT     price,* FROM     default.sum_of_volumes,     default.big_changes                    | ["default.big_changes_mv","default.sum_of_volumes_mv"]                |
+
+```
+
+In this example results above `default.big_changes_mv` and `default.sum_of_volumes_mv` are both materialized views.