bill=# create table tbl (id int, c1 int, c2 int, c3 int, c4 int);
CREATE TABLE

bill=# insert into tbl select generate_series(1,10000000), random()*100, random()*100, random()*100, random()*100;  
INSERT 0 10000000  

bill=# insert into tbl select generate_series(10000001,11000000), 200,200,200,200;  
INSERT 0 1000000  

bill=# create index idx_tbl_1 on tbl(id);  
CREATE INDEX  
bill=# create index idx_tbl_2 on tbl(c1,c2,c3,c4);  
CREATE INDEX  

bill=# vacuum analyze tbl;  
VACUUM  

bill=# explain select * from tbl where c1=200 and c2=200 order by id;
                                     QUERY PLAN
-------------------------------------------------------------------------------------
 Sort  (cost=72109.20..72344.16 rows=93984 width=20)
   Sort Key: id
   ->  Bitmap Heap Scan on tbl  (cost=1392.77..60811.81 rows=93984 width=20)
         Recheck Cond: ((c1 = 200) AND (c2 = 200))
         ->  Bitmap Index Scan on idx_tbl_2  (cost=0.00..1369.28 rows=93984 width=0)
               Index Cond: ((c1 = 200) AND (c2 = 200))
(6 rows)

bill=# begin;
BEGIN
bill=*# explain declare tt cursor for select * from tbl where c1=200 and c2=200 order by id;
                                  QUERY PLAN
-------------------------------------------------------------------------------
 Index Scan using idx_tbl_1 on tbl  (cost=0.43..329277.60 rows=93984 width=20)
   Filter: ((c1 = 200) AND (c2 = 200))
(2 rows)

導(dǎo)致選擇了錯(cuò)誤的執(zhí)行計(jì)劃。
	/* Determine what fraction of the plan is likely to be scanned */
	if (cursorOptions & CURSOR_OPT_FAST_PLAN)
	{
		/*
		 * We have no real idea how many tuples the user will ultimately FETCH
		 * from a cursor, but it is often the case that he doesn't want 'em
		 * all, or would prefer a fast-start plan anyway so that he can
		 * process some of the tuples sooner.  Use a GUC parameter to decide
		 * what fraction to optimize for.
		 */
		tuple_fraction = cursor_tuple_fraction;
		/*
		 * We document cursor_tuple_fraction as simply being a fraction, which
		 * means the edge cases 0 and 1 have to be treated specially here.  We
		 * convert 1 to 0 ("all the tuples") and 0 to a very small fraction.
		 */
		if (tuple_fraction >= 1.0)
			tuple_fraction = 0.0;
		else if (tuple_fraction <= 0.0)
			tuple_fraction = 1e-10;
	}
	else
	{
		/* Default assumption is we need all the tuples */
		tuple_fraction = 0.0;
	}