Query performance discontinuity

pgsql-performers,

Just out of curiosity, anybody with any ideas on what happens to this
query when the limit is 59626? It's as though 59626 = infinity?

pganalysis=> explain analyze select * from ps2 where tstart<> '2000-1-1
00:00:00' and time_stamp > '2000-1-1 00:00:00' order by
tstart,time_stamp limit 59624;
NOTICE: QUERY PLAN:

Limit (cost=0.00..160328.37 rows=59624 width=179) (actual
time=14.52..2225.16 rows=59624 loops=1)
-> Index Scan using ps2_idx on ps2 (cost=0.00..881812.85 rows=327935
width=179) (actual time=14.51..2154.59 rows=59625 loops=1)
Total runtime: 2265.93 msec

EXPLAIN
pganalysis=> explain analyze select * from ps2 where tstart<> '2000-1-1
00:00:00' and time_stamp > '2000-1-1 00:00:00' order by
tstart,time_stamp limit 59625;
NOTICE: QUERY PLAN:

Limit (cost=0.00..160331.06 rows=59625 width=179) (actual
time=0.45..2212.19 rows=59625 loops=1)
-> Index Scan using ps2_idx on ps2 (cost=0.00..881812.85 rows=327935
width=179) (actual time=0.45..2140.87 rows=59626 loops=1)
Total runtime: 2254.50 msec

EXPLAIN
pganalysis=> explain analyze select * from ps2 where tstart<> '2000-1-1
00:00:00' and time_stamp > '2000-1-1 00:00:00' order by
tstart,time_stamp limit 59626;
NOTICE: QUERY PLAN:

Limit (cost=160332.32..160332.32 rows=59626 width=179) (actual
time=37359.41..37535.85 rows=59626 loops=1)
-> Sort (cost=160332.32..160332.32 rows=327935 width=179) (actual
time=37359.40..37471.07 rows=59627 loops=1)
-> Seq Scan on ps2 (cost=0.00..13783.52 rows=327935 width=179)
(actual time=0.26..12433.00 rows=327960 loops=1)
Total runtime: 38477.39 msec

EXPLAIN
pganalysis=> explain analyze select * from ps2 where tstart<> '2000-1-1
00:00:00' and time_stamp > '2000-1-1 00:00:00' order by
tstart,time_stamp limit 59627;
NOTICE: QUERY PLAN:

Limit (cost=160332.32..160332.32 rows=59627 width=179) (actual
time=38084.85..38260.88 rows=59627 loops=1)
-> Sort (cost=160332.32..160332.32 rows=327935 width=179) (actual
time=38084.83..38194.63 rows=59628 loops=1)
-> Seq Scan on ps2 (cost=0.00..13783.52 rows=327935 width=179)
(actual time=0.15..12174.74 rows=327960 loops=1)
Total runtime: 38611.83 msec

EXPLAIN

pganalysis=> \d ps2
Table "ps2"
Column | Type | Modifiers
-------------+--------------------------+-----------
host | character varying(255) |
pid | integer |
line | integer |
time_stamp | timestamp with time zone |
seq | integer |
cpu_sys | real |
cpu_elapsed | real |
cpu_user | real |
cpu_syst | real |
cpu_usert | real |
mssp | integer |
sigp | integer |
msrt | integer |
msst | integer |
sigt | integer |
msrp | integer |
swap | integer |
swat | integer |
recp | integer |
rect | integer |
pgfp | integer |
pgft | integer |
icsp | integer |
vcst | integer |
icst | integer |
vcsp | integer |
fsbop | integer |
fsbos | integer |
fsbip | integer |
fsbis | integer |
dread | integer |
dwrit | integer |
sbhr | real |
sread | integer |
swrit | integer |
lbhr | real |
lread | integer |
lwrit | integer |
dbuser | character(8) |
tstart | timestamp with time zone |
Indexes: ps2_idx

pganalysis=> \d ps2_idx
Index "ps2_idx"
Column | Type
------------+--------------------------
tstart | timestamp with time zone
time_stamp | timestamp with time zone
btree

pganalysis=>

psql (PostgreSQL) 7.2
contains support for: readline, history, multibyte

Platform: Celeron 1.3GHz, 512MB 40GB IDE hard disk, Linux 2.4.8-26mdk
kernel

Regards,

Mike