postgresql - to write a SQL query which select rows where column value changed from previous row - 


create table status( id serial not null, site_id integer, sigplan smallint, server_time timestamp without time zone constraint status_data_pkey primary key (id)) (oids=false); alter table traffview.status_data owner postgres;

index: traffview.status_data_idx

create index status_data_idx on traffview.status_data using btree (server_time, site_id);

i have table this

id   siteid    sigplan    server_time  1    8300        1      2011-01-01          2    8300        1      2011-01-02           3    8300        2      2011-01-03           4    9600        1      2011-01-04           5    9600        2      2011-01-05

how select rows sigplan changed previous row siteid?

in example above, query should return rows 

2011-01-03 (sigplan changed 1 2 between 2011-01-01 , 2011-01-03 8300),  2011-01-05(sigplan changed 1 2 between 2011-01-04 , 2011-01-05 9600).
  • the table contains lot of data query should optimized.

select siteid, sigplan, max(server_time) traffview.status_data  group siteid, sigplan    having count(1) > 1 , max(server_time) > 'xxxxx' , max(server_time) < 'xxxxx'

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