Track shared buffer hits in pg_stat_io

Among other things, this should make it easier to calculate a useful cache hit
ratio by excluding buffer reads via buffer access strategies. As buffer access
strategies reuse buffers (and thus evict the prior buffer contents), it is
normal to see reads on repeated scans of the same data.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAAKRu_beMa9Hzih40%3DXPYqhDVz6tsgUGTrhZXRo%3Dunp%2Bszb%3DUA%40mail.gmail.com

3 files changed, 68 insertions, 5 deletions

diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index c71758db46..ab1aebfde4 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -1884,11 +1884,12 @@ pg_stat_io| SELECT backend_type,
     writes,
     extends,
     op_bytes,
+    hits,
     evictions,
     reuses,
     fsyncs,
     stats_reset
-   FROM pg_stat_get_io() b(backend_type, io_object, io_context, reads, writes, extends, op_bytes, evictions, reuses, fsyncs, stats_reset);
+   FROM pg_stat_get_io() b(backend_type, io_object, io_context, reads, writes, extends, op_bytes, hits, evictions, reuses, fsyncs, stats_reset);
 pg_stat_progress_analyze| SELECT s.pid,
     s.datid,
     d.datname,
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 55b4c6df01..5f1821938d 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -1131,6 +1131,7 @@ SELECT pg_stat_get_subscription_stats(NULL);
 -- - writes of shared buffers to permanent storage
 -- - extends of relations using shared buffers
 -- - fsyncs done to ensure the durability of data dirtying shared buffers
+-- - shared buffer hits
 -- There is no test for blocks evicted from shared buffers, because we cannot
 -- be sure of the state of shared buffers at the point the test is run.
 -- Create a regular table and insert some data to generate IOCONTEXT_NORMAL
@@ -1208,6 +1209,47 @@ SELECT :io_sum_shared_after_reads > :io_sum_shared_before_reads;
  t
 (1 row)
 
+SELECT sum(hits) AS io_sum_shared_before_hits
+  FROM pg_stat_io WHERE io_context = 'normal' AND io_object = 'relation' \gset
+-- Select from the table again to count hits.
+-- Ensure we generate hits by forcing a nested loop self-join with no
+-- materialize node. The outer side's buffer will stay pinned, preventing its
+-- eviction, while we loop through the inner side and generate hits.
+BEGIN;
+SET LOCAL enable_nestloop TO on; SET LOCAL enable_mergejoin TO off;
+SET LOCAL enable_hashjoin TO off; SET LOCAL enable_material TO off;
+-- ensure plan stays as we expect it to
+EXPLAIN (COSTS OFF) SELECT COUNT(*) FROM test_io_shared t1 INNER JOIN test_io_shared t2 USING (a);
+                QUERY PLAN                 
+-------------------------------------------
+ Aggregate
+   ->  Nested Loop
+         Join Filter: (t1.a = t2.a)
+         ->  Seq Scan on test_io_shared t1
+         ->  Seq Scan on test_io_shared t2
+(5 rows)
+
+SELECT COUNT(*) FROM test_io_shared t1 INNER JOIN test_io_shared t2 USING (a);
+ count 
+-------
+   100
+(1 row)
+
+COMMIT;
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT sum(hits) AS io_sum_shared_after_hits
+  FROM pg_stat_io WHERE io_context = 'normal' AND io_object = 'relation' \gset
+SELECT :io_sum_shared_after_hits > :io_sum_shared_before_hits;
+ ?column? 
+----------
+ t
+(1 row)
+
 DROP TABLE test_io_shared;
 -- Test that the follow IOCONTEXT_LOCAL IOOps are tracked in pg_stat_io:
 -- - eviction of local buffers in order to reuse them
@@ -1342,7 +1384,7 @@ SELECT pg_stat_have_stats('io', 0, 0);
  t
 (1 row)
 
-SELECT sum(evictions) + sum(reuses) + sum(extends) + sum(fsyncs) + sum(reads) + sum(writes) AS io_stats_pre_reset
+SELECT sum(evictions) + sum(reuses) + sum(extends) + sum(fsyncs) + sum(reads) + sum(writes) + sum(hits) AS io_stats_pre_reset
   FROM pg_stat_io \gset
 SELECT pg_stat_reset_shared('io');
  pg_stat_reset_shared 
@@ -1350,7 +1392,7 @@ SELECT pg_stat_reset_shared('io');
  
 (1 row)
 
-SELECT sum(evictions) + sum(reuses) + sum(extends) + sum(fsyncs) + sum(reads) + sum(writes) AS io_stats_post_reset
+SELECT sum(evictions) + sum(reuses) + sum(extends) + sum(fsyncs) + sum(reads) + sum(writes) + sum(hits) AS io_stats_post_reset
   FROM pg_stat_io \gset
 SELECT :io_stats_post_reset < :io_stats_pre_reset;
  ?column? 
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index d958e70a86..58db803ed6 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -541,6 +541,7 @@ SELECT pg_stat_get_subscription_stats(NULL);
 -- - writes of shared buffers to permanent storage
 -- - extends of relations using shared buffers
 -- - fsyncs done to ensure the durability of data dirtying shared buffers
+-- - shared buffer hits
 
 -- There is no test for blocks evicted from shared buffers, because we cannot
 -- be sure of the state of shared buffers at the point the test is run.
@@ -587,6 +588,25 @@ SELECT pg_stat_force_next_flush();
 SELECT sum(reads) AS io_sum_shared_after_reads
   FROM pg_stat_io WHERE io_context = 'normal' AND io_object = 'relation'  \gset
 SELECT :io_sum_shared_after_reads > :io_sum_shared_before_reads;
+
+SELECT sum(hits) AS io_sum_shared_before_hits
+  FROM pg_stat_io WHERE io_context = 'normal' AND io_object = 'relation' \gset
+-- Select from the table again to count hits.
+-- Ensure we generate hits by forcing a nested loop self-join with no
+-- materialize node. The outer side's buffer will stay pinned, preventing its
+-- eviction, while we loop through the inner side and generate hits.
+BEGIN;
+SET LOCAL enable_nestloop TO on; SET LOCAL enable_mergejoin TO off;
+SET LOCAL enable_hashjoin TO off; SET LOCAL enable_material TO off;
+-- ensure plan stays as we expect it to
+EXPLAIN (COSTS OFF) SELECT COUNT(*) FROM test_io_shared t1 INNER JOIN test_io_shared t2 USING (a);
+SELECT COUNT(*) FROM test_io_shared t1 INNER JOIN test_io_shared t2 USING (a);
+COMMIT;
+SELECT pg_stat_force_next_flush();
+SELECT sum(hits) AS io_sum_shared_after_hits
+  FROM pg_stat_io WHERE io_context = 'normal' AND io_object = 'relation' \gset
+SELECT :io_sum_shared_after_hits > :io_sum_shared_before_hits;
+
 DROP TABLE test_io_shared;
 
 -- Test that the follow IOCONTEXT_LOCAL IOOps are tracked in pg_stat_io:
@@ -674,10 +694,10 @@ SELECT :io_sum_bulkwrite_strategy_extends_after > :io_sum_bulkwrite_strategy_ext
 
 -- Test IO stats reset
 SELECT pg_stat_have_stats('io', 0, 0);
-SELECT sum(evictions) + sum(reuses) + sum(extends) + sum(fsyncs) + sum(reads) + sum(writes) AS io_stats_pre_reset
+SELECT sum(evictions) + sum(reuses) + sum(extends) + sum(fsyncs) + sum(reads) + sum(writes) + sum(hits) AS io_stats_pre_reset
   FROM pg_stat_io \gset
 SELECT pg_stat_reset_shared('io');
-SELECT sum(evictions) + sum(reuses) + sum(extends) + sum(fsyncs) + sum(reads) + sum(writes) AS io_stats_post_reset
+SELECT sum(evictions) + sum(reuses) + sum(extends) + sum(fsyncs) + sum(reads) + sum(writes) + sum(hits) AS io_stats_post_reset
   FROM pg_stat_io \gset
 SELECT :io_stats_post_reset < :io_stats_pre_reset;