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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
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45
src/test/isolation/specs/vacuum-reltuples.spec
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45
src/test/isolation/specs/vacuum-reltuples.spec
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# Test for vacuum's handling of reltuples when pages are skipped due
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# to page pins. We absolutely need to avoid setting reltuples=0 in
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# such cases, since that interferes badly with planning.
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setup {
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create table smalltbl
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as select i as id from generate_series(1,20) i;
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alter table smalltbl set (autovacuum_enabled = off);
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}
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setup {
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vacuum analyze smalltbl;
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}
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teardown {
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drop table smalltbl;
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}
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session "worker"
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step "open" {
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begin;
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declare c1 cursor for select * from smalltbl;
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}
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step "fetch1" {
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fetch next from c1;
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}
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step "close" {
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commit;
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}
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step "stats" {
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select relpages, reltuples from pg_class
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where oid='smalltbl'::regclass;
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}
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session "vacuumer"
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step "vac" {
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vacuum smalltbl;
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}
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step "modify" {
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insert into smalltbl select max(id)+1 from smalltbl;
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delete from smalltbl where id in (select min(id) from smalltbl);
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}
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permutation "modify" "vac" "stats"
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permutation "modify" "open" "fetch1" "vac" "close" "stats"
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permutation "modify" "vac" "stats"
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