Cursors and different settings for default_statistics_target

Hi,

the following statement retrieves 16358 rows in a cursor by fetching
1024 rows in bulks on a 8.2.4 server:

DECLARE curs_285058224 CURSOR FOR SELECT objid, attrid, aggrid, lineid,
objval FROM atobjval WHERE objid IN
(281479288456304,281479288456359,281479288456360,281479288456384,2814792
88456385,281479288456403,281479288456404,281479288456406,281479288456408
,281479288456432,281479288456433,281479288456434,281479288456438,2814792
88456442,281479288456468,281479288456499,281479288456546,281479288456547
,281479288456590,281479288456636,281479288456638,281479288456722,2814792
88457111,281479288457125,281479288457126,281479288457143,281479288457229
,281479288457230,281479288457477,281479288457478,281479288457546,2814792
88457559,281479288457676,281479288457686,281479288457792,281479288457808
,281479288457809,281479288457852,281479288457853,281479288457902,2814792
88457961,281479288457962,281479288458005,281479288458097,281479288458116
,281479288458155,281479288458156,281479288458183,281479288458516,2814792
88458523,281479288458576,281479288458577,281479288458624,281479288458716
,281479288458721,281479288458735,281479288458736,281479288458737,2814792
88458758,281479288458786,281479288458788,281479288458789,281479288458794
,281479288458806,281479288458914,281479288458957,281479288458958,2814792
88459029,281479288459126,281479288459127,281479288459135,281479288459259
,281479288459260,281479288459261,281479288459262,281479288459321,2814792
88459425,281479288459426,281479288459427,281479288459428,281479288459447
,281479288459450,281479288459453,281479288459457,281479288459462,2814792
88459607,281479288459608,281479288459635,281479288459636,281479288459732
,281479288459767,281479288459954,281479288459974,281479288459975,2814792
88459976,281479288459977,281479288460034,281479288460060,281479288460070
,281479288460073,281479288460088,281479288460162,281479288460163,2814792
88460167,281479288460170,281479288460173,281479288460176,281479288460179
,281479288460182,281479288460185,281479288460188,281479288460217,2814792
88460290,281479288460292,281479288460318,281479288460325,281479288460332
,281479288460337,281479288460339,281479288460377,281479288460378,2814792
88460394,281479288460412,281479288460457,281479288460565,281479288460566
,281479288460567,281479288460608,281479288460609,281479288460683,2814792
88460684,281479288461021,281479288461024,281479288461059,281479288461091
,281479288461281,281479288461367,281479288461368,281479288461369,2814792
88461377,281479288461429,281479288461477,281479288461483,281479288461484
,281479288461485,281479288461493,281479288461494,281479288461502,2814792
88461522,281479288461570,281479288461578,281479288461654,281479288461655
,281479288461690,281479288461711,281479288461712,281479288461747,2814792
88461776,281479288461777,281479288461838,281479288461839,281479288461878
,281479288461889,281479288462036,281479288462083,281479288462090,2814792
88462096,281479288462104,281479288462129,281479288462136,281479288462276
,281479288462277,281479288462366,281479288462367,281479288462448,2814792
88462450,281479288462502,281479288462817,281479288462967,281479288462968
,281479288462969,281479288463200,281479288463246,281479288463247,2814792
88463248,281479288463255,281479288463437,281479288463441,281479288463462
,281479288463482,281479288463642,281479288463645,281479288463782,2814792
88463790,281479288463802,281479288463809,281479288463819,281479288463843
,281479288463859,281479288463967,281479288463968,281479288463969,2814792
88465253,281479288465396,281479288465397,281479288465417,281479288465429
,281479288465436,281479288467191,285774255752169,285774255752181,2857742
55752183,285774255752188,285774255752189,285774255752198,285774255753788
,285774255753789,285774255753790,285774255753793,285774255753794,2857742
55753808,285774255753809,285774255753811,285774255753812,285774255753828
,285774255753893,285774255753993,285774255754091,285774255754106,2857742
55754110,285774255754160,285774255755169,285774255755179,285774255755184
,285774255755187,285774255755205,285774255755252,285774255755254,2857742
55755271,285774255755481,285774255755494,285774255755514,285774255755534
,285774255755571,285774255755597,285774255755616,285774255755622,2857742
55755632,285774255755696,285774255755717,285774255755729,285774255755747
,285774255755759,285774255755787,285774255755791,285774255755798,2857742
55755802,285774255757269,285774255757270,285774255757286,285774255757287
,285774255757518,285774255757687,285774255757797,285774255761019,2857742
55761021,285774255761069,285774255761070,285774255764181,285774255764182
,285774255764196,285774255764204,285774255764276,285774255764290,2857742
55764301,285774255764312,285774255764333,285774255764334,285774255764335
,285774255764367,285774255764369,285774255764371,285774255764382,2857742
55764394,285774255764418,285774255764420,285774255764430,285774255764486
,285774255764490,285774255764498,285774255764616,285774255764683,2857742
55764787,285774255764802,285774255765031,285774255765043,285774255765052
,285774255765066,285774255765081,285774255765145,285774255766471,2857742
55767469,285774255767809,285774255767971,285774255768111,285774255768151
,285774255768193,285774255768199,285774255768220,285774255769244,2857742
55769317,285774255770269,285774255770343,285774255770373,285774255770374
,285774255770475,285774255770488,285774255772471,285774255773974,2857742
55773977,285774255773981,285774255774003,285774255774012,285774255774018
,285774255774079,285774255774080,285774255774098,285774255774106,2857742
55774110,285774255774130,285774255774775,285774255777173,285774255777188
,285774255777205,285774255777219,285774255777241,285774255777242,2857742
55777243,285774255777245,285774255777260,285774255777299,285774255777337
,285774255777422,285774255777445,285774255777446,285774255778669,2857742
55778671,285774255778672,285774255779069,285774255779070,285774255781196
,285774255782209,285774255782221,285774255782224,285774255782226,2857742
55782325,285774255782430,285774255783469,285774255783470,285774255783575
,285774255783576,285774255783577,285774255785169,285774255785170,2857742
55785173,285774255785174,285774255785177,285774255785178,285774255785189
,285774255785190,285774255785197,285774255785198,285774255785209,2857742
55785210,285774255785238,285774255785239,285774255788781,285774255788784
,285774255788821,285774255788827,285774255788830,285774255788852,2857742
55788867,285774255788889,285774255789671,285774255789852,285774255790150
,285774255790369,285774255790370,285774255790373,285774255790569,2857742
55790571,285774255790572,285774255790573,285774255790645,285774255790655
,285774255793470,285774255793517,285774255793647,285774255793650,2857742
55793687,285774255795211,285774255797003,285774255798195,285774255798234
,285774255798242,285774255800551,285774255800689,285774255800696,2857742
55800751,285774255800821,285774255809954,285774255809981,285774255810032
,285774255810033,285774255812694,285774255812706,285774255812708,2857742
55812713,285774255812746,285774255812747,285774255812752,285774255812761
,285774255812765,285774255812768,285774255812771,285774255812774,2857742
55812857,285774255813980,285774255814277,285774255814296,285774255814313
,285774255814314,285774255814333,285774255814357,285774255814368,2857742
55814385,285774255815169,285774255816279,285774255816675,285774255817669
,285774255817688,285774255817699,285774255817793,285774255817874,2857742
55817952,285774255817960,285774255817981,285774255818045,285774255818052
,285774255818067,285774255818068,285774255818106,285774255820418,2857742
55821169,285774255821224,285774255821232,285774255821303,285774255821387
,285774255821393,285774255821468,285774255821481,285774255826269,2857742
55826980,285774255826985,285774255826994,285774255827971,285774255827999
,285774255828050,285774255828168,285774255828171,285774255828172,2857742
55828173,285774255828174,285774255828205,285774255828213,285774255828221
,285774255832360,285774255832381,285774255832500,285774255832534,2857742
55832551,285774255832590,285774255832611,285774255832641,285774255832726
,285774255832782,285774255833369,285774255833477,285774255833490,2857742
55833532,285774255833537,285774255833669,285774255834874,285774255834950
,285774255835014,285774255835035,285774255836198,285774255836199,2857742
55837674) ORDER BY objid, attrid, aggrid, lineid;

When we use 20 as default_statistics_target the retrieval of the data
takes 7.5 seconds - with 25 as default_statistics_target (with restart
and analyze) it takes 0.6 seconds.
The query plan is identical in both situations (row estimation differs a
little bit) - the query is always fast when it is executed without a
cursor.

The problem is the last of the 16 fetches - it takes about 7 seconds
when using 20 as default_statistics_target.
During the fetch there is no disk IO - the backend which is running the
query uses 100 % of one cpu core during the seven seconds.

The following shows OProfile output for the seven seconds:
samples % symbol name
200318 41.0421 ExecEvalScalarArrayOp
106214 21.7616 int8eq
36031 7.3822 ExecMakeFunctionResultNoSets
17894 3.6662 ExecEvalAnd
16518 3.3843 ExecEvalScalarVar
13282 2.7213 ExecEvalConst
9980 2.0448 HeapTupleSatisfiesSnapshot
9696 1.9866 SyncOneBuffer
8643 1.7708 slot_getattr
6792 1.3916 int8le
6232 1.2768 hash_search_with_hash_value
6032 1.2359 heap_release_fetch
5690 1.1658 ExecEvalOr
5442 1.1150 _bt_checkkeys
5430 1.1125 LWLockAcquire
5228 1.0711 PinBuffer

Does anyone has an explanation for this behavior?

Regards,
Robert Hell