idc danny wrote:
Hi James,
Than you for your response.
What I want to achieve is to give to the application
user 10k rows where the records are one after another
in the table, and the application has a paginating GUI
("First page", "Previous page", "Next page", "Last
page" - all links & "Jump to page" combobox) where
thsi particular query gets to run if the user clicks
on the "Last page" link.
The application receive the first 10k rows in under a
second when the user clicks on "First page" link and
receive the last 10k rows in about 60 seconds when he
clicks on "Last page" link.
You need a sequence that automatically assigns an ascending "my_rownum" to each
row as it is added to the table, and an index on that my_rownum column. Then you select
your page by (for example)
select * from my_table where my_rownum >= 100 and id < 110;
That will do what you want, with instant performance that's linear over your
whole table.
If your table will have deletions, then you have to update the row numbering a lot, which will cause you
terrible performance problems due to the nature of the UPDATE operation in Postgres. If this is the case,
then you should keep a separate table just for numbering the rows, which is joined to your main table when
you want to retrieve a "page" of data. When you delete data (which should be batched, since this
will be expensive), then you truncate your rownum table, reset the sequence that generates your row numbers,
then regenerate your row numbers with something like "insert into my_rownum_table (select id,
nextval('my_rownum_seq') from my_big_table)". To retrieve a page, just do "select ... from
my_table join my_rownum_table on (...)", which will be really fast since you'll have indexes on both
tables.
Note that this method requires that you have a primary key, or at least a
unique column, on your main table, so that you have something to join with your
row-number table.
Craig
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