>>>>> "Donald" == Donald Dong <xd...@csumb.edu> writes:
Donald> Hi, Donald> I created some empty tables and run ` EXPLAIN ANALYZE` on Donald> `SELECT * `. I found the results have different row numbers, Donald> but the tables are all empty. Empty tables are something of a special case, because the planner doesn't assume that they will _stay_ empty, and using an estimate of 0 or 1 rows would tend to create a distorted plan that would likely blow up in runtime as soon as you insert a second row. The place to look for info would be estimate_rel_size in optimizer/util/plancat.c, from which you can see that empty tables get a default size estimate of 10 pages. Thus: Donald> =# CREATE TABLE t1(id INT, data INT); Donald> =# EXPLAIN ANALYZE SELECT * FROM t1; Donald> Seq Scan on t1 (cost=0.00..32.60 rows=2260 width=8) (actual Donald> time=0.003..0.003 rows=0 loops=1) An (int,int) tuple takes about 36 bytes, so you can get about 226 of them on a page, so 10 pages is 2260 rows. Donald> =# CREATE TABLE t2(data VARCHAR); Donald> =# EXPLAIN ANALYZE SELECT * FROM t2; Donald> Seq Scan on t2 (cost=0.00..23.60 rows=1360 width=32) (actual Donald> time=0.002..0.002 rows=0 loops=1) Size of a varchar with no specified length isn't known, so the planner determines an average length of 32 by the time-honoured method of rectal extraction (see get_typavgwidth in lsyscache.c), making 136 rows per page. -- Andrew (irc:RhodiumToad)
