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Commit1fe6668

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Attempt to stabilize flapping regression test
Per buildfarm animal mylodon, the plan for this test was sometimesswapping the join order for tenk1 and tenk2. Given that add_path() hasno code that would cause this fluctuation when given paths with consistentcosts, this indicates that the costs must be fluctuating in some runs.The only proven reason I've seen where that could happen was slightvariations in pg_class.relpages for some tables. This was demonstrated tobe true byf03a9ca and related discussion. Manually adjusting tenk2'spg_class.relpages by subtracting just 1 page does cause the plan to changefor this test.Here we've not gone to the same lengths to prove that's what's going onin this case. Proving that does not seem worth the time. Let's justshrink one side of the join so the additional cost of the swapped joinorder is sufficiently different that if the relpages estimate is off a fewpages that the planner still shouldn't swap the join order.Reported-by: Thomas MunroAuthor: Andy Fan, David RowleyDiscussion:https://postgr.es/m/CA+hUKGLqC-NobKYfjxNM3Gexv9OJ-Fhvy9bugUcXsZjTqH7W=Q@mail.gmail.com
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‎src/test/regress/expected/union.out

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@@ -1438,7 +1438,7 @@ where (x = 0) or (q1 >= q2 and q1 <= q2);
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-- Ensure we get a Nested Loop join between tenk1 and tenk2
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explain (costs off)
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select t1.unique1 from tenk1 t1
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inner join tenk2 t2 on t1.tenthous = t2.tenthous
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inner join tenk2 t2 on t1.tenthous = t2.tenthous and t2.thousand = 0
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union all
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(values(1)) limit 1;
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QUERY PLAN
@@ -1450,8 +1450,9 @@ inner join tenk2 t2 on t1.tenthous = t2.tenthous
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-> Seq Scan on tenk1 t1
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-> Materialize
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-> Seq Scan on tenk2 t2
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Filter: (thousand = 0)
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-> Result
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(8 rows)
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(9 rows)
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-- Ensure there is no problem if cheapest_startup_path is NULL
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explain (costs off)

‎src/test/regress/sql/union.sql

Lines changed: 1 addition & 1 deletion
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@@ -548,7 +548,7 @@ where (x = 0) or (q1 >= q2 and q1 <= q2);
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-- Ensure we get a Nested Loop join between tenk1 and tenk2
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explain (costs off)
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selectt1.unique1from tenk1 t1
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inner join tenk2 t2ont1.tenthous=t2.tenthous
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inner join tenk2 t2ont1.tenthous=t2.tenthousandt2.thousand=0
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union all
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(values(1))limit1;
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