From 7b69611755ac3b9132710d83a1139b4c5606578d Mon Sep 17 00:00:00 2001
From: Christopher Baines <mail@cbaines.net>
Date: Tue, 29 Nov 2022 11:09:55 +0000
Subject: [PATCH] Expose metrics from pg_stats

---
 guix-data-service/metrics.scm        | 20 ++++++++++++++-
 guix-data-service/web/controller.scm | 37 ++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/guix-data-service/metrics.scm b/guix-data-service/metrics.scm
index 7af8924..7ec6b8e 100644
--- a/guix-data-service/metrics.scm
+++ b/guix-data-service/metrics.scm
@@ -20,7 +20,8 @@
   #:use-module (squee)
   #:export (fetch-high-level-table-size-metrics
             fetch-pg-stat-user-tables-metrics
-            fetch-pg-stat-user-indexes-metrics))
+            fetch-pg-stat-user-indexes-metrics
+            fetch-pg-stats-metrics))
 
 (define (fetch-high-level-table-size-metrics conn)
   ;; Adapted from https://wiki.postgresql.org/wiki/Disk_Usage
@@ -155,3 +156,20 @@ WHERE pg_stat_user_indexes.schemaname = 'guix_data_service'")
         (idx-tup-fetch . ,idx_tup_fetch)
         (bytes         . ,size_in_bytes))))
    (exec-query conn query)))
+
+(define (fetch-pg-stats-metrics conn)
+  (define query
+    "
+SELECT tablename, attname, null_frac, n_distinct, correlation
+FROM pg_stats
+WHERE schemaname = 'guix_data_service'")
+
+  (map
+   (match-lambda
+     ((tablename attname null_frac n_distinct correlation)
+      `((table-name  . ,tablename)
+        (column-name . ,attname)
+        (null-frac   . ,(string->number null_frac))
+        (n-distinct  . ,(string->number n_distinct))
+        (correlation . ,(and=> correlation string->number)))))
+   (exec-query conn query)))
diff --git a/guix-data-service/web/controller.scm b/guix-data-service/web/controller.scm
index 7aac071..988145e 100644
--- a/guix-data-service/web/controller.scm
+++ b/guix-data-service/web/controller.scm
@@ -151,6 +151,22 @@
                                            #:labels '(name))))
                                        pg-stat-indexes-fields))
 
+         (pg-stats-fields '(null-frac n-distinct correlation))
+
+         (pg-stats-metrics (map (lambda (field)
+                                  (cons
+                                   field
+                                   (make-gauge-metric
+                                    registry
+                                    (string-append
+                                     "pg_stats_"
+                                     (string-replace-substring
+                                      (symbol->string field)
+                                      "-"
+                                      "_"))
+                                    #:labels '(table column))))
+                                pg-stats-fields))
+
          (gc-metrics-updater
           (get-gc-metrics-updater registry)))
 
@@ -167,6 +183,9 @@
                 (pg-stat-user-indexes-metrics
                  (with-thread-postgresql-connection
                   fetch-pg-stat-user-indexes-metrics))
+                (pg-stats-metric-values
+                 (with-thread-postgresql-connection
+                  fetch-pg-stats-metrics))
                 (load-new-guix-revision-job-metrics
                  (with-thread-postgresql-connection
                   select-load-new-guix-revision-job-metrics)))
@@ -228,6 +247,24 @@
                   field-values)))
              pg-stat-user-indexes-metrics)
 
+        (map (lambda (field-values)
+               (let ((table (assq-ref field-values 'table-name))
+                     (column (assq-ref field-values 'column-name)))
+                 (for-each
+                  (match-lambda
+                    (('table-name . _) #f)
+                    (('column-name . _) #f)
+                    ((_ . #f) #f)
+                    ((field . value)
+                     (let ((metric (or (assq-ref pg-stats-metrics field)
+                                       (error field))))
+                       (metric-set metric
+                                   value
+                                   #:label-values `((table . ,table)
+                                                    (column . ,column))))))
+                  field-values)))
+             pg-stats-metric-values)
+
         (for-each (match-lambda
                     ((repository-label state count)
                      (metric-set