Avoid having vacuum set reltuples to 0 on non-empty relations in the

presence of page pins, which leads to serious estimation errors in the
planner.  This particularly affects small heavily-accessed tables,
especially where locking (e.g. from FK constraints) forces frequent
vacuums for mxid cleanup.

Fix by keeping separate track of pages whose live tuples were actually
counted vs. pages that were only scanned for freezing purposes.  Thus,
reltuples can only be set to 0 if all pages of the relation were
actually counted.

Backpatch to all supported versions.

Per bug #14057 from Nicolas Baccelli, analyzed by me.

Discussion: https://postgr.es/m/20160331103739.8956.94469@wrigleys.postgresql.org

3 files changed, 108 insertions, 0 deletions

diff --git a/src/test/isolation/expected/vacuum-reltuples.out b/src/test/isolation/expected/vacuum-reltuples.out
new file mode 100644
index 00000000000..ee3adf58050
--- /dev/null
+++ b/src/test/isolation/expected/vacuum-reltuples.out
@@ -0,0 +1,62 @@
+Parsed test spec with 2 sessions
+
+starting permutation: modify vac stats
+step modify: 
+    insert into smalltbl select max(id)+1 from smalltbl;
+    delete from smalltbl where id in (select min(id) from smalltbl);
+
+step vac: 
+    vacuum smalltbl;
+
+step stats: 
+    select relpages, reltuples from pg_class
+     where oid='smalltbl'::regclass;
+
+relpages       reltuples      
+
+1              20             
+
+starting permutation: modify open fetch1 vac close stats
+step modify: 
+    insert into smalltbl select max(id)+1 from smalltbl;
+    delete from smalltbl where id in (select min(id) from smalltbl);
+
+step open: 
+    begin;
+    declare c1 cursor for select * from smalltbl;
+
+step fetch1: 
+    fetch next from c1;
+
+id             
+
+2              
+step vac: 
+    vacuum smalltbl;
+
+step close: 
+    commit;
+
+step stats: 
+    select relpages, reltuples from pg_class
+     where oid='smalltbl'::regclass;
+
+relpages       reltuples      
+
+1              20             
+
+starting permutation: modify vac stats
+step modify: 
+    insert into smalltbl select max(id)+1 from smalltbl;
+    delete from smalltbl where id in (select min(id) from smalltbl);
+
+step vac: 
+    vacuum smalltbl;
+
+step stats: 
+    select relpages, reltuples from pg_class
+     where oid='smalltbl'::regclass;
+
+relpages       reltuples      
+
+1              20             
diff --git a/src/test/isolation/isolation_schedule b/src/test/isolation/isolation_schedule
index 2606a276247..8e404b7a355 100644
--- a/src/test/isolation/isolation_schedule
+++ b/src/test/isolation/isolation_schedule
@@ -56,4 +56,5 @@ test: alter-table-2
 test: alter-table-3
 test: create-trigger
 test: async-notify
+test: vacuum-reltuples
 test: timeouts
diff --git a/src/test/isolation/specs/vacuum-reltuples.spec b/src/test/isolation/specs/vacuum-reltuples.spec
new file mode 100644
index 00000000000..52bc405547a
--- /dev/null
+++ b/src/test/isolation/specs/vacuum-reltuples.spec
@@ -0,0 +1,45 @@
+# Test for vacuum's handling of reltuples when pages are skipped due
+# to page pins. We absolutely need to avoid setting reltuples=0 in
+# such cases, since that interferes badly with planning.
+
+setup {
+    create table smalltbl
+        as select i as id from generate_series(1,20) i;
+    alter table smalltbl set (autovacuum_enabled = off);
+}
+setup {
+    vacuum analyze smalltbl;
+}
+
+teardown {
+    drop table smalltbl;
+}
+
+session "worker"
+step "open" {
+    begin;
+    declare c1 cursor for select * from smalltbl;
+}
+step "fetch1" {
+    fetch next from c1;
+}
+step "close" {
+    commit;
+}
+step "stats" {
+    select relpages, reltuples from pg_class
+     where oid='smalltbl'::regclass;
+}
+
+session "vacuumer"
+step "vac" {
+    vacuum smalltbl;
+}
+step "modify" {
+    insert into smalltbl select max(id)+1 from smalltbl;
+    delete from smalltbl where id in (select min(id) from smalltbl);
+}
+
+permutation "modify" "vac" "stats"
+permutation "modify" "open" "fetch1" "vac" "close" "stats"
+permutation "modify" "vac" "stats"