Use actual backend IDs in pg_stat_get_backend_idset() and friends.

Up to now, the ID values returned by pg_stat_get_backend_idset() and
used by pg_stat_get_backend_activity() and allied functions were just
indexes into a local array of sessions seen by the last stats refresh.
This is problematic for a few reasons.  The "ID" of a session can vary
over its existence, which is surprising.  Also, while these numbers
often match the "backend ID" used for purposes like temp schema
assignment, that isn't reliably true.  We can fairly cheaply switch
things around to make these numbers actually be the sessions' backend
IDs.  The added test case illustrates that with this definition, the
temp schema used by a given session can be obtained given its PID.

While here, delete some dead code that guarded against getting
a NULL return from pgstat_fetch_stat_local_beentry().  That can't
happen as long as the caller is careful to pass an in-range array
index, as all the callers are.  (This code may not have been dead
when written, but it surely is now.)

Nathan Bossart

Discussion: https://postgr.es/m/20220815205811.GA250990@nathanxps13

2 files changed, 21 insertions, 4 deletions

diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 6a10dc462bd..f701da20697 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -576,9 +576,9 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 
 -- Test pg_stat_bgwriter checkpointer-related stats, together with pg_stat_wal
 SELECT checkpoints_req AS rqst_ckpts_before FROM pg_stat_bgwriter \gset
--- Test pg_stat_wal
+-- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
-CREATE TABLE test_stats_temp AS SELECT 17;
+CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
 -- of the checkpoint. But after a second checkpoint we'll see at least the
@@ -597,6 +597,17 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+-- Test pg_stat_get_backend_idset() and some allied functions.
+-- In particular, verify that their notion of backend ID matches
+-- our temp schema index.
+SELECT (current_schemas(true))[1] = ('pg_temp_' || beid::text) AS match
+FROM pg_stat_get_backend_idset() beid
+WHERE pg_stat_get_backend_pid(beid) = pg_backend_pid();
+ match 
+-------
+ t
+(1 row)
+
 -----
 -- Test that resetting stats works for reset timestamp
 -----
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index a6b0e9e0428..eb081f65a42 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -303,10 +303,10 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 -- Test pg_stat_bgwriter checkpointer-related stats, together with pg_stat_wal
 SELECT checkpoints_req AS rqst_ckpts_before FROM pg_stat_bgwriter \gset
 
--- Test pg_stat_wal
+-- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
-CREATE TABLE test_stats_temp AS SELECT 17;
+CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -318,6 +318,12 @@ CHECKPOINT;
 SELECT checkpoints_req > :rqst_ckpts_before FROM pg_stat_bgwriter;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+-- Test pg_stat_get_backend_idset() and some allied functions.
+-- In particular, verify that their notion of backend ID matches
+-- our temp schema index.
+SELECT (current_schemas(true))[1] = ('pg_temp_' || beid::text) AS match
+FROM pg_stat_get_backend_idset() beid
+WHERE pg_stat_get_backend_pid(beid) = pg_backend_pid();
 
 -----
 -- Test that resetting stats works for reset timestamp