Limit changes query plan

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Hi all,
I'm using 8.2.6 and I'm observing a trange behaviour using
offset and limits.

This are the two queries that are puzzling me:

explain SELECT c.id, tsk, lir, nctr, nctn, ncts, rvel,ecp, pvcp, pvcc,pvcf,pvcl,ldcn,ogtd,sgti
FROM t_OA_2_00_card c JOIN t_OA_2_00_dt dt ON (dt.card_id = c.id)
WHERE ecp=18 AND _to >= 1500 AND _from <= 1550
ORDER BY nctr,nctn,ncts,rvel
offset 0 ;
QUERY PLAN
- ----------------------------------------------------------------------------------------------------------------
Limit (cost=175044.75..175071.04 rows=10518 width=90)
-> Sort (cost=175044.75..175071.04 rows=10518 width=90)
Sort Key: c.nctr, c.nctn, c.ncts, c.rvel
-> Hash Join (cost=25830.72..174342.12 rows=10518 width=90)
Hash Cond: (c.id = dt.card_id)
-> Bitmap Heap Scan on t_oa_2_00_card c (cost=942.36..148457.19 rows=101872 width=90)
Recheck Cond: (ecp = 18)
-> Bitmap Index Scan on i7_t_oa_2_00_card (cost=0.00..916.89 rows=101872 width=0)
Index Cond: (ecp = 18)
-> Hash (cost=22743.45..22743.45 rows=171593 width=8)
-> Bitmap Heap Scan on t_oa_2_00_dt dt (cost=2877.26..22743.45 rows=171593 width=8)
Recheck Cond: (_from <= 1550)
Filter: (_to >= 1500)
-> Bitmap Index Scan on i_oa_2_00_dt_from (cost=0.00..2834.36 rows=182546 width=0)
Index Cond: (_from <= 1550)

explain SELECT c.id, tsk, lir, nctr, nctn, ncts, rvel,ecp, pvcp, pvcc,pvcf,pvcl,ldcn,ogtd,sgti
FROM t_OA_2_00_card c JOIN t_OA_2_00_dt dt ON (dt.card_id = c.id)
WHERE ecp=18 AND _to >= 1500 AND _from <= 1550
ORDER BY nctr,nctn,ncts,rvel
offset 0 limit 5;
QUERY PLAN
- --------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..2125.12 rows=5 width=90)
-> Nested Loop (cost=0.00..4470402.02 rows=10518 width=90)
-> Index Scan using i_oa_2_00_card_keys on t_oa_2_00_card c (cost=0.00..3927779.56 rows=101872 width=90)
Filter: (ecp = 18)
-> Index Scan using i_oa_2_00_dt_for on t_oa_2_00_dt dt (cost=0.00..5.31 rows=1 width=8)
Index Cond: (dt.card_id = c.id)
Filter: ((_to >= 1500) AND (_from <= 1550))

using the limit I have an execution time of minutes vs a some seconds.

What am I missing here ?

Regards
Gaetano Mendola

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