#476 Occasional failure of cluster regression test

[Koichi Suzuki]

Cluster regession test fails with the following message. It does not occur in serial_schedule but in parallel_schedule.

! ERROR:  could not read block 0 in file "base/16384/24994": read only 0 of 8192 bytes                                  
  -- cluster a single table using the indisclustered bit previously set                                                 
  DELETE FROM clstr_1;                                                                                                  
+ ERROR:  could not read block 0 in file "base/16384/24994": read only 0 of 8192 bytes                                  
  INSERT INTO clstr_1 VALUES (2);                                                                                       
+ ERROR:  duplicate key value violates unique constraint "clstr_1_pkey"                                                 
+ DETAIL:  Key (a)=(2) already exists.                                                                                  
  INSERT INTO clstr_1 VALUES (1);                                                                                       
+ ERROR:  duplicate key value violates unique constraint "clstr_1_pkey"                                                 
+ DETAIL:  Key (a)=(1) already exists.                                                                                  
  CLUSTER clstr_1;                                                                                                      
+ ERROR:  could not read block 0 in file "base/16384/24994": read only 0 of 8192         bytes                                  
  SELECT * FROM clstr_1                                                                                                 
  ORDER BY 1;                                                                                                           
! ERROR:  could not read block 0 in file "base/16384/24994": read only 0 of 8192 bytes                                  
  -- Test MVCC-safety of cluster. There isn't much we can do to verify the