Remove replacement selection sort.

At the time replacement_sort_tuples was introduced, there were still
cases where replacement selection sort noticeably outperformed using
quicksort even for the first run.  However, those cases seem to have
evaporated as a result of further improvements made since that time
(and perhaps also advances in CPU technology).  So remove replacement
selection and the controlling GUC entirely.  This makes tuplesort.c
noticeably simpler and probably paves the way for further
optimizations someone might want to do later.

Peter Geoghegan, with review and testing by Tomas Vondra and me.

Discussion: https://postgr.es/m/CAH2-WzmmNjG_K0R9nqYwMq3zjyJJK+hCbiZYNGhAy-Zyjs64GQ@mail.gmail.com

2 files changed, 2 insertions, 29 deletions

diff --git a/src/test/regress/expected/cluster.out b/src/test/regress/expected/cluster.out
index 097ac2b0060..82713bfa2c7 100644
--- a/src/test/regress/expected/cluster.out
+++ b/src/test/regress/expected/cluster.out
@@ -444,22 +444,8 @@ create table clstr_4 as select * from tenk1;
 create index cluster_sort on clstr_4 (hundred, thousand, tenthous);
 -- ensure we don't use the index in CLUSTER nor the checking SELECTs
 set enable_indexscan = off;
--- Use external sort that only ever uses quicksort to sort runs:
+-- Use external sort:
 set maintenance_work_mem = '1MB';
-set replacement_sort_tuples = 0;
-cluster clstr_4 using cluster_sort;
-select * from
-(select hundred, lag(hundred) over () as lhundred,
-        thousand, lag(thousand) over () as lthousand,
-        tenthous, lag(tenthous) over () as ltenthous from clstr_4) ss
-where row(hundred, thousand, tenthous) <= row(lhundred, lthousand, ltenthous);
- hundred | lhundred | thousand | lthousand | tenthous | ltenthous 
----------+----------+----------+-----------+----------+-----------
-(0 rows)
-
--- Replacement selection will now be forced.  It should only produce a single
--- run, due to the fact that input is found to be presorted:
-set replacement_sort_tuples = 150000;
 cluster clstr_4 using cluster_sort;
 select * from
 (select hundred, lag(hundred) over () as lhundred,
@@ -472,7 +458,6 @@ where row(hundred, thousand, tenthous) <= row(lhundred, lthousand, ltenthous);
 
 reset enable_indexscan;
 reset maintenance_work_mem;
-reset replacement_sort_tuples;
 -- clean up
 DROP TABLE clustertest;
 DROP TABLE clstr_1;
diff --git a/src/test/regress/sql/cluster.sql b/src/test/regress/sql/cluster.sql
index 8dd9459bda0..a6c2757efaa 100644
--- a/src/test/regress/sql/cluster.sql
+++ b/src/test/regress/sql/cluster.sql
@@ -203,19 +203,8 @@ create index cluster_sort on clstr_4 (hundred, thousand, tenthous);
 -- ensure we don't use the index in CLUSTER nor the checking SELECTs
 set enable_indexscan = off;
 
--- Use external sort that only ever uses quicksort to sort runs:
+-- Use external sort:
 set maintenance_work_mem = '1MB';
-set replacement_sort_tuples = 0;
-cluster clstr_4 using cluster_sort;
-select * from
-(select hundred, lag(hundred) over () as lhundred,
-        thousand, lag(thousand) over () as lthousand,
-        tenthous, lag(tenthous) over () as ltenthous from clstr_4) ss
-where row(hundred, thousand, tenthous) <= row(lhundred, lthousand, ltenthous);
-
--- Replacement selection will now be forced.  It should only produce a single
--- run, due to the fact that input is found to be presorted:
-set replacement_sort_tuples = 150000;
 cluster clstr_4 using cluster_sort;
 select * from
 (select hundred, lag(hundred) over () as lhundred,
@@ -225,7 +214,6 @@ where row(hundred, thousand, tenthous) <= row(lhundred, lthousand, ltenthous);
 
 reset enable_indexscan;
 reset maintenance_work_mem;
-reset replacement_sort_tuples;
 
 -- clean up
 DROP TABLE clustertest;