SELECT attname, n_distinct, most_common_vals FROM pg_stats WHERE tablename = 'lieferliste';
....

fixstop |          1 | {f}
# Es sind also nur 'false-Werte' in der DB gespeichert.

kis2_prod=# EXPLAIN select * from lieferliste where fixstop='t';
                                  QUERY PLAN
------------------------------------------------------------------------------
 Index Scan using idx_test on lieferliste  (cost=0.00..2.01 rows=1 width=118)
   Index Cond: (fixstop = true)
# Bei einer Query nach 't' erkennt er anhand der Statistic die Selektivität von 't' - und geht über den Index - soweit ok

kis2_prod=# EXPLAIN select * from lieferliste where fixstop='f';
                              QUERY PLAN
-----------------------------------------------------------------------
 Seq Scan on lieferliste  (cost=0.00..55892.88 rows=1379510 width=118)
   Filter: (fixstop = false)
(2 Zeilen)
# Bei einer Query nach 'f' erkennt er anhand der Statistic die Wertlosigkeit der WHERE-Clause - und macht korrekt einen Full Table Scan.


EXPLAIN select * from lieferliste where fixstop;
                           QUERY PLAN
-----------------------------------------------------------------
 Seq Scan on lieferliste  (cost=0.00..52444.10 rows=1 width=118)
   Filter: fixstop
# Das hier versteh ich nicht.

psql test
Willkommen bei psql 7.4.2, dem interaktiven PostgreSQL-Terminal.

Geben Sie ein:  \copyright für Urheberrechtsinformationen
                \h für Hilfe über SQL-Anweisungen
                \? für Hilfe über interne Anweisungen
                \g oder Semikolon, um eine Anfrage auszuführen
                \q um zu beenden

test=# \d
         Liste der Relationen
 Schema | Name |   Typ   | Eigentümer
--------+------+---------+-------------
 public | a    | Tabelle | grueni
(1 Zeile)

test=# CREATE TABLE laaang ( id serial primary key not null, bool boolean not null default 'f');
NOTICE:  CREATE TABLE will create implicit sequence "laaang_id_seq" for "serial" column "laaang.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "laaang_pkey" for table "laaang"
CREATE TABLE
test=# INSERT INTO laaang(wert) values ('f');
ERROR:  column "wert" of relation "laaang" does not exist
test=# INSERT INTO laaang(bool) values ('f');
INSERT 17180 1
test=# INSERT INTO laaang(bool) values ('f');
INSERT 17181 1
test=# INSERT INTO laaang(bool) values ('f');
INSERT 17182 1
test=# INSERT INTO laaang(bool) values ('f');
INSERT 17183 1
test=# INSERT INTO laaang(bool) values ('f');
INSERT 17184 1
test=# SELECT count(*) from pg_statistic ;
 count
-------
   245
(1 Zeile)

test=# INSERT INTO laaang (bool) SELECT 'f' from pg_statistic a, pg_statistic b;

INSERT 0 60025
test=#
test=# SELECT count(*) from laaang;
 count
-------
 60030
(1 Zeile)

test=# INSERT INTO laaang(bool) SELECT bool from laaang;
INSERT 0 60030
test=# INSERT INTO laaang(bool) SELECT bool from laaang;
INSERT 0 120060
test=# VACUUM ANALYZE ;
VACUUM

test=# CREATE INDEX idx_laang on laaang (bool);
CREATE INDEX
test=# EXPLAIN SELECT * from laaang where bool='t';
                               QUERY PLAN
------------------------------------------------------------------------
 Index Scan using idx_laang on laaang  (cost=0.00..2.01 rows=1 width=5)
   Index Cond: (bool = true)
(2 Zeilen)
--- So soll es sein !

test=# EXPLAIN SELECT * from laaang where bool;
                       QUERY PLAN
---------------------------------------------------------
 Seq Scan on laaang  (cost=0.00..3579.20 rows=1 width=5)
   Filter: bool
(2 Zeilen)

-- So soll es nicht sein!