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Bad plan choice nestloop vs. hashjoin

From: Kenneth Marshall <ktm(at)rice(dot)edu>
To: pgsql-performance(at)postgresql(dot)org
Subject: Bad plan choice nestloop vs. hashjoin
Date: 2010-01-15 22:54:16
Message-ID: 20100115225416.GO1895@it.is.rice.edu (view raw or flat)
Thread:
Lists: pgsql-performance
Dear performance group:

We have just upgraded our monitoring server software and
now the following query for graphing the data performs
abysmally with the default settings. Here is the results
of the EXPLAIN ANALYZE run with nestloops enabled:

SET enable_nestloop = 'on';
EXPLAIN SELECT g.graphid FROM graphs g,graphs_items gi,items i,hosts_groups hg,rights r,users_groups ug WHERE  (g.graphid/100000000000000) in (0)  AND gi.graphid=g.graphid AND i.itemid=gi.itemid AND hg.hostid=i.hostid AND r.id=hg.groupid  AND r.groupid=ug.usrgrpid AND ug.userid=20 AND r.permission>=2 AND NOT EXISTS(  SELECT gii.graphid  FROM graphs_items gii, items ii  WHERE gii.graphid=g.graphid  AND gii.itemid=ii.itemid  AND EXISTS(  SELECT hgg.groupid  FROM hosts_groups hgg, rights rr, users_groups ugg  WHERE ii.hostid=hgg.hostid  AND rr.id=hgg.groupid  AND rr.groupid=ugg.usrgrpid  AND ugg.userid=20 AND rr.permission<2)) AND  (g.graphid IN (2,3,4,5,386,387,969,389,971,972,973,446,447,448,449,450,451,471,456,470,473,477,472,474,475,476,478,479,480,481,482,483,484,459,614,655,658,645,490,492,489,493,496,495,498,497,499,501,500,502,974,558,559,562,566,563,564,565,567,568,569,570,571,535,572,573,534,536,538,539,540,541,542,543,544,545,537,546,547,548,552,553,554,555,556,549,550,551,557,577,578,579,580,574,576,581,835,587,588,589,590,560,561,836,591,592,593,594,595,827,389,495,498,497,597,598,599,975,978,999,1004,604,605,606,679,616,634,635,636,637,638,618,629,630,631,632,633,671,682,669,670,678,679,680,674,672,676,673,675,677,681,682,683,683,644,652,829,681,687,698,685,686,705,706,707,708,830,945,946,710,716,712,714,713,709,718,721,720,719,723,724,747,749,750,730,731,732,733,734,735,736,737,738,739,740,741,742,743,744,745,772,774,775,755,756,757,758,759,760,761,762,763,764,765,766,767,768,769,770,777,776,977,824,823,826,825,829,832,833,835,581,836,842,852,854,839,840,838,853,855,847,848,944,846,859,850,899,901,902,903,864,865,866,867,976,979,939,941,942,943,906,907,908,909,910,868,969,991,950,955,964,966,952,953,962,965,967,959,961,968,1001,1002,1003,986,987,988,994,995,996,1008,1006,1007,1009,1010));
-----

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    QUERY PLAN                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=94.07..10304.00 rows=1 width=8) (actual time=194.557..27975.338 rows=607 loops=1)
   Join Filter: (r.groupid = ug.usrgrpid)
   ->  Seq Scan on users_groups ug  (cost=0.00..1.15 rows=1 width=8) (actual time=0.020..0.026 rows=1 loops=1)
         Filter: (userid = 20)
   ->  Nested Loop  (cost=94.07..10302.65 rows=16 width=16) (actual time=98.126..27965.748 rows=5728 loops=1)
         ->  Nested Loop  (cost=94.07..10301.76 rows=2 width=16) (actual time=98.085..27933.529 rows=928 loops=1)
               ->  Nested Loop  (cost=94.07..10301.20 rows=2 width=16) (actual time=98.074..27924.076 rows=837 loops=1)
                     ->  Nested Loop  (cost=94.07..10299.07 rows=2 width=16) (actual time=98.063..27914.106 rows=837 loops=1)
                           ->  Nested Loop Anti Join  (cost=94.07..10294.64 rows=1 width=8) (actual time=98.049..27907.702 rows=281 loops=1)
                                 Join Filter: (gii.graphid = g.graphid)
                                 ->  Bitmap Heap Scan on graphs g  (cost=94.07..233.17 rows=1 width=8) (actual time=0.529..1.772 rows=281 loops=1)
                                       Recheck Cond: (graphid = ANY ('{2,3,4,5,386,387,969,389,971,972,973,446,447,448,449,450,451,471,456,470,473,477,472,474,475,476,478,479,480,481,482,483,484,459,614,655,658,645,490,492,489,493,496,495,498,497,499,501,500,502,974,558,559,562,566,563,564,565,567,568,569,570,571,535,572,573,534,536,538,539,540,541,542,543,544,545,537,546,547,548,552,553,554,555,556,549,550,551,557,577,578,579,580,574,576,581,835,587,588,589,590,560,561,836,591,592,593,594,595,827,389,495,498,497,597,598,599,975,978,999,1004,604,605,606,679,616,634,635,636,637,638,618,629,630,631,632,633,671,682,669,670,678,679,680,674,672,676,673,675,677,681,682,683,683,644,652,829,681,687,698,685,686,705,706,707,708,830,945,946,710,716,712,714,713,709,718,721,720,719,723,724,747,749,750,730,731,732,733,734,735,736,737,738,739,740,741,742,743,744,745,772,774,775,755,756,757,758,759,760,761,762,763,764,765,766,767,768,769,770,777,776,977,824,823,826,825,829,832,833,835,581,836,842,852,854,839,840,838,853,855,847,848,944,846,859,850,899,901,902,903,864,865,866,867,976,979,939,941,942,943,906,907,908,909,910,868,969,991,950,955,964,966,952,953,962,965,967,959,961,968,1001,1002,1003,986,987,988,994,995,996,1008,1006,1007,1009,1010}'::bigint[]))
                                       Filter: ((graphid / 100000000000000::bigint) = 0)
                                       ->  Bitmap Index Scan on graphs_pkey  (cost=0.00..94.07 rows=246 width=0) (actual time=0.507..0.507 rows=294 loops=1)
                                             Index Cond: (graphid = ANY ('{2,3,4,5,386,387,969,389,971,972,973,446,447,448,449,450,451,471,456,470,473,477,472,474,475,476,478,479,480,481,482,483,484,459,614,655,658,645,490,492,489,493,496,495,498,497,499,501,500,502,974,558,559,562,566,563,564,565,567,568,569,570,571,535,572,573,534,536,538,539,540,541,542,543,544,545,537,546,547,548,552,553,554,555,556,549,550,551,557,577,578,579,580,574,576,581,835,587,588,589,590,560,561,836,591,592,593,594,595,827,389,495,498,497,597,598,599,975,978,999,1004,604,605,606,679,616,634,635,636,637,638,618,629,630,631,632,633,671,682,669,670,678,679,680,674,672,676,673,675,677,681,682,683,683,644,652,829,681,687,698,685,686,705,706,707,708,830,945,946,710,716,712,714,713,709,718,721,720,719,723,724,747,749,750,730,731,732,733,734,735,736,737,738,739,740,741,742,743,744,745,772,774,775,755,756,757,758,759,760,761,762,763,764,765,766,767,768,769,770,777,776,977,824,823,826,825,829,832,833,835,581,836,842,852,854,839,840,838,853,855,847,848,944,846,859,850,899,901,902,903,864,865,866,867,976,979,939,941,942,943,906,907,908,909,910,868,969,991,950,955,964,966,952,953,962,965,967,959,961,968,1001,1002,1003,986,987,988,994,995,996,1008,1006,1007,1009,1010}'::bigint[]))
                                 ->  Nested Loop  (cost=0.00..17449.43 rows=1954 width=8) (actual time=99.304..99.304 rows=0 loops=281)
                                       ->  Index Scan using graphs_items_2 on graphs_items gii  (cost=0.00..69.83 rows=1954 width=16) (actual time=0.013..3.399 rows=1954 loops=281)
                                       ->  Index Scan using items_pkey on items ii  (cost=0.00..8.88 rows=1 width=8) (actual time=0.046..0.046 rows=0 loops=549074)
                                             Index Cond: (ii.itemid = gii.itemid)
                                             Filter: (alternatives: SubPlan 1 or hashed SubPlan 2)
                                             SubPlan 1
                                               ->  Nested Loop  (cost=0.00..7.83 rows=1 width=0) (actual time=0.040..0.040 rows=0 loops=549074)
                                                     Join Filter: (rr.groupid = ugg.usrgrpid)
                                                     ->  Nested Loop  (cost=0.00..6.67 rows=1 width=8) (actual time=0.037..0.037 rows=0 loops=549074)
                                                           Join Filter: (hgg.groupid = rr.id)
                                                           ->  Index Scan using hosts_groups_1 on hosts_groups hgg  (cost=0.00..4.27 rows=1 width=8) (actual time=0.003..0.005 rows=1 loops=549074)
                                                                 Index Cond: ($0 = hostid)
                                                           ->  Seq Scan on rights rr  (cost=0.00..2.39 rows=1 width=16) (actual time=0.027..0.027 rows=0 loops=532214)
                                                                 Filter: (rr.permission < 2)
                                                     ->  Seq Scan on users_groups ugg  (cost=0.00..1.15 rows=1 width=8) (never executed)
                                                           Filter: (ugg.userid = 20)
                                             SubPlan 2
                                               ->  Nested Loop  (cost=2.34..8.13 rows=1 width=8) (never executed)
                                                     ->  Nested Loop  (cost=0.00..3.55 rows=1 width=8) (never executed)
                                                           Join Filter: (rr.groupid = ugg.usrgrpid)
                                                           ->  Seq Scan on rights rr  (cost=0.00..2.39 rows=1 width=16) (never executed)
                                                                 Filter: (permission < 2)
                                                           ->  Seq Scan on users_groups ugg  (cost=0.00..1.15 rows=1 width=8) (never executed)
                                                                 Filter: (ugg.userid = 20)
                                                     ->  Bitmap Heap Scan on hosts_groups hgg  (cost=2.34..4.45 rows=11 width=16) (never executed)
                                                           Recheck Cond: (hgg.groupid = rr.id)
                                                           ->  Bitmap Index Scan on hosts_groups_2  (cost=0.00..2.33 rows=11 width=0) (never executed)
                                                                 Index Cond: (hgg.groupid = rr.id)
                           ->  Index Scan using graphs_items_2 on graphs_items gi  (cost=0.00..4.41 rows=2 width=16) (actual time=0.005..0.010 rows=3 loops=281)
                                 Index Cond: (gi.graphid = g.graphid)
                     ->  Index Scan using items_pkey on items i  (cost=0.00..1.05 rows=1 width=16) (actual time=0.004..0.006 rows=1 loops=837)
                           Index Cond: (i.itemid = gi.itemid)
               ->  Index Scan using hosts_groups_1 on hosts_groups hg  (cost=0.00..0.27 rows=1 width=16) (actual time=0.003..0.005 rows=1 loops=837)
                     Index Cond: (hg.hostid = i.hostid)
         ->  Index Scan using rights_2 on rights r  (cost=0.00..0.38 rows=5 width=16) (actual time=0.004..0.015 rows=6 loops=928)
               Index Cond: (r.id = hg.groupid)
               Filter: (r.permission >= 2)
 Total runtime: 27976.516 ms
(53 rows)

And here is the  the same plan with nestloops disabled:

SET enable_nestloop = 'off';
EXPLAIN ANALYZE SELECT g.graphid FROM graphs g,graphs_items gi,items i,hosts_groups hg,rights r,users_groups ug WHERE  (g.graphid/100000000000000) in (0)  AND gi.graphid=g.graphid AND i.itemid=gi.itemid AND hg.hostid=i.hostid AND r.id=hg.groupid  AND r.groupid=ug.usrgrpid AND ug.userid=20 AND r.permission>=2 AND NOT EXISTS(  SELECT gii.graphid  FROM graphs_items gii, items ii  WHERE gii.graphid=g.graphid  AND gii.itemid=ii.itemid  AND EXISTS(  SELECT hgg.groupid  FROM hosts_groups hgg, rights rr, users_groups ugg  WHERE ii.hostid=hgg.hostid  AND rr.id=hgg.groupid  AND rr.groupid=ugg.usrgrpid  AND ugg.userid=20 AND rr.permission<2)) AND  (g.graphid IN (2,3,4,5,386,387,969,389,971,972,973,446,447,448,449,450,451,471,456,470,473,477,472,474,475,476,478,479,480,481,482,483,484,459,614,655,658,645,490,492,489,493,496,495,498,497,499,501,500,502,974,558,559,562,566,563,564,565,567,568,569,570,571,535,572,573,534,536,538,539,540,541,542,543,544,545,537,546,547,548,552,553,554,555,556,549,550,551,557,577,578,579,580,574,576,581,835,587,588,589,590,560,561,836,591,592,593,594,595,827,389,495,498,497,597,598,599,975,978,999,1004,604,605,606,679,616,634,635,636,637,638,618,629,630,631,632,633,671,682,669,670,678,679,680,674,672,676,673,675,677,681,682,683,683,644,652,829,681,687,698,685,686,705,706,707,708,830,945,946,710,716,712,714,713,709,718,721,720,719,723,724,747,749,750,730,731,732,733,734,735,736,737,738,739,740,741,742,743,744,745,772,774,775,755,756,757,758,759,760,761,762,763,764,765,766,767,768,769,770,777,776,977,824,823,826,825,829,832,833,835,581,836,842,852,854,839,840,838,853,855,847,848,944,846,859,850,899,901,902,903,864,865,866,867,976,979,939,941,942,943,906,907,908,909,910,868,969,991,950,955,964,966,952,953,962,965,967,959,961,968,1001,1002,1003,986,987,988,994,995,996,1008,1006,1007,1009,1010));
-----

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             QUERY PLAN                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Hash Join  (cost=106463.65..106466.57 rows=1 width=8) (actual time=67.513..68.637 rows=607 loops=1)
   Hash Cond: (r.id = hg.groupid)
   ->  Hash Join  (cost=1.16..4.05 rows=8 width=8) (actual time=0.255..0.416 rows=15 loops=1)
         Hash Cond: (r.groupid = ug.usrgrpid)
         ->  Seq Scan on rights r  (cost=0.00..2.39 rows=111 width=16) (actual time=0.015..0.184 rows=111 loops=1)
               Filter: (permission >= 2)
         ->  Hash  (cost=1.15..1.15 rows=1 width=8) (actual time=0.022..0.022 rows=1 loops=1)
               ->  Seq Scan on users_groups ug  (cost=0.00..1.15 rows=1 width=8) (actual time=0.010..0.015 rows=1 loops=1)
                     Filter: (userid = 20)
   ->  Hash  (cost=106462.46..106462.46 rows=2 width=16) (actual time=67.225..67.225 rows=928 loops=1)
         ->  Hash Join  (cost=106457.61..106462.46 rows=2 width=16) (actual time=63.608..65.720 rows=928 loops=1)
               Hash Cond: (hg.hostid = i.hostid)
               ->  Seq Scan on hosts_groups hg  (cost=0.00..4.06 rows=206 width=16) (actual time=0.008..0.305 rows=206 loops=1)
               ->  Hash  (cost=106457.58..106457.58 rows=2 width=16) (actual time=63.565..63.565 rows=837 loops=1)
                     ->  Hash Anti Join  (cost=105576.85..106457.58 rows=2 width=16) (actual time=19.955..62.166 rows=837 loops=1)
                           Hash Cond: (g.graphid = gii.graphid)
                           ->  Hash Join  (cost=282.09..1162.79 rows=2 width=16) (actual time=13.021..52.860 rows=837 loops=1)
                                 Hash Cond: (i.itemid = gi.itemid)
                                 ->  Seq Scan on items i  (cost=0.00..831.13 rows=13213 width=16) (actual time=0.004..22.955 rows=13213 loops=1)
                                 ->  Hash  (cost=282.07..282.07 rows=2 width=16) (actual time=9.890..9.890 rows=837 loops=1)
                                       ->  Hash Join  (cost=233.18..282.07 rows=2 width=16) (actual time=1.514..8.514 rows=837 loops=1)
                                             Hash Cond: (gi.graphid = g.graphid)
                                             ->  Seq Scan on graphs_items gi  (cost=0.00..41.54 rows=1954 width=16) (actual time=0.005..2.713 rows=1954 loops=1)
                                             ->  Hash  (cost=233.17..233.17 rows=1 width=8) (actual time=1.489..1.489 rows=281 loops=1)
                                                   ->  Bitmap Heap Scan on graphs g  (cost=94.07..233.17 rows=1 width=8) (actual time=0.526..1.056 rows=281 loops=1)
                                                         Recheck Cond: (graphid = ANY ('{2,3,4,5,386,387,969,389,971,972,973,446,447,448,449,450,451,471,456,470,473,477,472,474,475,476,478,479,480,481,482,483,484,459,614,655,658,645,490,492,489,493,496,495,498,497,499,501,500,502,974,558,559,562,566,563,564,565,567,568,569,570,571,535,572,573,534,536,538,539,540,541,542,543,544,545,537,546,547,548,552,553,554,555,556,549,550,551,557,577,578,579,580,574,576,581,835,587,588,589,590,560,561,836,591,592,593,594,595,827,389,495,498,497,597,598,599,975,978,999,1004,604,605,606,679,616,634,635,636,637,638,618,629,630,631,632,633,671,682,669,670,678,679,680,674,672,676,673,675,677,681,682,683,683,644,652,829,681,687,698,685,686,705,706,707,708,830,945,946,710,716,712,714,713,709,718,721,720,719,723,724,747,749,750,730,731,732,733,734,735,736,737,738,739,740,741,742,743,744,745,772,774,775,755,756,757,758,759,760,761,762,763,764,765,766,767,768,769,770,777,776,977,824,823,826,825,829,832,833,835,581,836,842,852,854,839,840,838,853,855,847,848,944,846,859,850,899,901,902,903,864,865,866,867,976,979,939,941,942,943,906,907,908,909,910,868,969,991,950,955,964,966,952,953,962,965,967,959,961,968,1001,1002,1003,986,987,988,994,995,996,1008,1006,1007,1009,1010}'::bigint[]))
                                                         Filter: ((graphid / 100000000000000::bigint) = 0)
                                                         ->  Bitmap Index Scan on graphs_pkey  (cost=0.00..94.07 rows=246 width=0) (actual time=0.504..0.504 rows=294 loops=1)
                                                               Index Cond: (graphid = ANY ('{2,3,4,5,386,387,969,389,971,972,973,446,447,448,449,450,451,471,456,470,473,477,472,474,475,476,478,479,480,481,482,483,484,459,614,655,658,645,490,492,489,493,496,495,498,497,499,501,500,502,974,558,559,562,566,563,564,565,567,568,569,570,571,535,572,573,534,536,538,539,540,541,542,543,544,545,537,546,547,548,552,553,554,555,556,549,550,551,557,577,578,579,580,574,576,581,835,587,588,589,590,560,561,836,591,592,593,594,595,827,389,495,498,497,597,598,599,975,978,999,1004,604,605,606,679,616,634,635,636,637,638,618,629,630,631,632,633,671,682,669,670,678,679,680,674,672,676,673,675,677,681,682,683,683,644,652,829,681,687,698,685,686,705,706,707,708,830,945,946,710,716,712,714,713,709,718,721,720,719,723,724,747,749,750,730,731,732,733,734,735,736,737,738,739,740,741,742,743,744,745,772,774,775,755,756,757,758,759,760,761,762,763,764,765,766,767,768,769,770,777,776,977,824,823,826,825,829,832,833,835,581,836,842,852,854,839,840,838,853,855,847,848,944,846,859,850,899,901,902,903,864,865,866,867,976,979,939,941,942,943,906,907,908,909,910,868,969,991,950,955,964,966,952,953,962,965,967,959,961,968,1001,1002,1003,986,987,988,994,995,996,1008,1006,1007,1009,1010}'::bigint[]))
                           ->  Hash  (cost=105270.33..105270.33 rows=1954 width=8) (actual time=6.914..6.914 rows=0 loops=1)
                                 ->  Hash Join  (cost=65.97..105270.33 rows=1954 width=8) (actual time=6.912..6.912 rows=0 loops=1)
                                       Hash Cond: (ii.itemid = gii.itemid)
                                       ->  Index Scan using items_pkey on items ii  (cost=0.00..105110.51 rows=6606 width=8) (actual time=6.907..6.907 rows=0 loops=1)
                                             Filter: (alternatives: SubPlan 1 or hashed SubPlan 2)
                                             SubPlan 1
                                               ->  Hash Join  (cost=3.56..7.86 rows=1 width=0) (never executed)
                                                     Hash Cond: (rr.groupid = ugg.usrgrpid)
                                                     ->  Hash Join  (cost=2.40..6.68 rows=1 width=8) (never executed)
                                                           Hash Cond: (hgg.groupid = rr.id)
                                                           ->  Index Scan using hosts_groups_1 on hosts_groups hgg  (cost=0.00..4.27 rows=1 width=8) (never executed)
                                                                 Index Cond: ($0 = hostid)
                                                           ->  Hash  (cost=2.39..2.39 rows=1 width=16) (never executed)
                                                                 ->  Seq Scan on rights rr  (cost=0.00..2.39 rows=1 width=16) (never executed)
                                                                       Filter: (permission < 2)
                                                     ->  Hash  (cost=1.15..1.15 rows=1 width=8) (never executed)
                                                           ->  Seq Scan on users_groups ugg  (cost=0.00..1.15 rows=1 width=8) (never executed)
                                                                 Filter: (userid = 20)
                                             SubPlan 2
                                               ->  Hash Join  (cost=3.58..8.56 rows=1 width=8) (actual time=0.062..0.062 rows=0 loops=1)
                                                     Hash Cond: (hgg.groupid = rr.id)
                                                     ->  Seq Scan on hosts_groups hgg  (cost=0.00..4.06 rows=206 width=16) (actual time=0.006..0.006 rows=1 loops=1)
                                                     ->  Hash  (cost=3.56..3.56 rows=1 width=8) (actual time=0.040..0.040 rows=0 loops=1)
                                                           ->  Hash Join  (cost=1.16..3.56 rows=1 width=8) (actual time=0.037..0.037 rows=0 loops=1)
                                                                 Hash Cond: (rr.groupid = ugg.usrgrpid)
                                                                 ->  Seq Scan on rights rr  (cost=0.00..2.39 rows=1 width=16) (actual time=0.035..0.035 rows=0 loops=1)
                                                                       Filter: (permission < 2)
                                                                 ->  Hash  (cost=1.15..1.15 rows=1 width=8) (never executed)
                                                                       ->  Seq Scan on users_groups ugg  (cost=0.00..1.15 rows=1 width=8) (never executed)
                                                                             Filter: (userid = 20)
                                       ->  Hash  (cost=41.54..41.54 rows=1954 width=16) (never executed)
                                             ->  Seq Scan on graphs_items gii  (cost=0.00..41.54 rows=1954 width=16) (never executed)
 Total runtime: 69.630 ms
(62 rows)


It looks like it thinks that there will only be 1 row hitting
the nestloop, when in fact there are 607 rows. Does anyone have
any ideas for how to coerce PostgreSQL into using the hashjoin
plan instead? I am running 8.4.2 and have done a database-wide
vacuum/analyze.

Regards,
Ken

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Next:From: Kevin GrittnerDate: 2010-01-15 22:58:57
Subject: Re: Bad plan choice nestloop vs. hashjoin
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Subject: Re: a heavy duty operation on an "unused" table kills my server

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