Reduce cardinality of pg_stat_statements

Make the example queries.yaml `pg_stat_statements` query safer.
* Select the top 10% of queries by total query time.
* Only expose the top 100 queries by total query time.
* Keep only the most useful metrics.

Fixes: #549

Signed-off-by: SuperQ <superq@gmail.com>

queries.yaml

@@ -148,75 +148,52 @@ pg_statio_user_tables:
 
 # WARNING: This set of metrics can be very expensive on a busy server as every unique query executed will create an additional time series
 pg_stat_statements:
-  query: "SELECT t2.rolname, t3.datname, queryid, calls, total_time / 1000 as total_time_seconds, min_time / 1000 as min_time_seconds, max_time / 1000 as max_time_seconds, mean_time / 1000 as mean_time_seconds, stddev_time / 1000 as stddev_time_seconds, rows, shared_blks_hit, shared_blks_read, shared_blks_dirtied, shared_blks_written, local_blks_hit, local_blks_read, local_blks_dirtied, local_blks_written, temp_blks_read, temp_blks_written, blk_read_time / 1000 as blk_read_time_seconds, blk_write_time / 1000 as blk_write_time_seconds FROM pg_stat_statements t1 JOIN pg_roles t2 ON (t1.userid=t2.oid) JOIN pg_database t3 ON (t1.dbid=t3.oid) WHERE t2.rolname != 'rdsadmin'"
-  master: true
+  query: |
+    SELECT
+      pg_get_userbyid(userid) as user,
+      pg_database.datname,
+      pg_stat_statements.queryid,
+      pg_stat_statements.calls as calls_total,
+      pg_stat_statements.total_time / 1000.0 as seconds_total,
+      pg_stat_statements.rows as rows_total,
+      pg_stat_statements.blk_read_time / 1000.0 as block_read_seconds_total,
+      pg_stat_statements.blk_write_time / 1000.0 as block_write_seconds_total
+      FROM pg_stat_statements
+      JOIN pg_database
+        ON pg_database.oid = pg_stat_statements.dbid
+      WHERE
+        total_time > (
+          SELECT percentile_cont(0.1)
+            WITHIN GROUP (ORDER BY total_time)
+            FROM pg_stat_statements
+        )
+      ORDER BY seconds_total DESC
+      LIMIT 100
   metrics:
-    - rolname:
+    - user:
         usage: "LABEL"
-        description: "Name of user"
+        description: "The user who executed the statement"
     - datname:
         usage: "LABEL"
-        description: "Name of database"
+        description: "The database in which the statement was executed"
     - queryid:
         usage: "LABEL"
-        description: "Query ID"
-    - calls:
+        description: "Internal hash code, computed from the statement's parse tree"
+    - calls_total:
         usage: "COUNTER"
         description: "Number of times executed"
-    - total_time_seconds:
+    - seconds_total:
         usage: "COUNTER"
-        description: "Total time spent in the statement, in milliseconds"
-    - min_time_seconds:
-        usage: "GAUGE"
-        description: "Minimum time spent in the statement, in milliseconds"
-    - max_time_seconds:
-        usage: "GAUGE"
-        description: "Maximum time spent in the statement, in milliseconds"
-    - mean_time_seconds:
-        usage: "GAUGE"
-        description: "Mean time spent in the statement, in milliseconds"
-    - stddev_time_seconds:
-        usage: "GAUGE"
-        description: "Population standard deviation of time spent in the statement, in milliseconds"
-    - rows:
+        description: "Total time spent in the statement, in seconds"
+    - rows_total:
         usage: "COUNTER"
         description: "Total number of rows retrieved or affected by the statement"
-    - shared_blks_hit:
-        usage: "COUNTER"
-        description: "Total number of shared block cache hits by the statement"
-    - shared_blks_read:
-        usage: "COUNTER"
-        description: "Total number of shared blocks read by the statement"
-    - shared_blks_dirtied:
-        usage: "COUNTER"
-        description: "Total number of shared blocks dirtied by the statement"
-    - shared_blks_written:
-        usage: "COUNTER"
-        description: "Total number of shared blocks written by the statement"
-    - local_blks_hit:
-        usage: "COUNTER"
-        description: "Total number of local block cache hits by the statement"
-    - local_blks_read:
-        usage: "COUNTER"
-        description: "Total number of local blocks read by the statement"
-    - local_blks_dirtied:
-        usage: "COUNTER"
-        description: "Total number of local blocks dirtied by the statement"
-    - local_blks_written:
-        usage: "COUNTER"
-        description: "Total number of local blocks written by the statement"
-    - temp_blks_read:
-        usage: "COUNTER"
-        description: "Total number of temp blocks read by the statement"
-    - temp_blks_written:
-        usage: "COUNTER"
-        description: "Total number of temp blocks written by the statement"
-    - blk_read_time_seconds:
+    - block_read_seconds_total:
         usage: "COUNTER"
-        description: "Total time the statement spent reading blocks, in milliseconds (if track_io_timing is enabled, otherwise zero)"
-    - blk_write_time_seconds:
+        description: "Total time the statement spent reading blocks, in seconds"
+    - block_write_seconds_total:
         usage: "COUNTER"
-        description: "Total time the statement spent writing blocks, in milliseconds (if track_io_timing is enabled, otherwise zero)"
+        description: "Total time the statement spent writing blocks, in seconds"
 
 pg_process_idle:
   query: |