Greetings, * Lutz Fischer (l.fischer@xxxxxxxx) wrote: > Data in [sp] are never changed. I can probably reduce the size by > changing datatypes from numeric to float but I was wondering if it > would be more efficient - primarily in terms of storage - to change > the structure to have two arrays in [s]. E.g. The short answer is 'probably', but it really depends on how wide your rows are. > I haven't really found anything yet how much space (e.g. how many > bytes) an array will use compared to a table row in postgresql. There's a 24-byte overhead for each tuple. If the width of the tuple's columns ends up being less than 24 bytes then half (or more) of the space used is for the tuple header. Arrays have a bit of overhead themsleves but are then densely packed. In testing that I've done, a table which looks like: CREATE TABLE t1 ( c1 int ); Will end up with a couple hundred rows per 8k page (perhaps 250 or so), meaning that you get ~1k of actual data for 8k of storage. Changing this to an array, like so: CREATE TABLE t1 ( c1 int[] ); And then storing 3-4 tuples per 8k page (keeping each tuple under the 2k TOAST limit) lead to being able to store something like 450 ints per tuple with a subsequent result of 1800 ints per page and ~7.2k worth of actual data for 8k of storage, which was much more efficient for storage. Of course, the tuple header is actually useful data in many environments- if you go with this approach then you have to work out how to deal with the fact that a given tuple is either visible or not, and all the ints in the array for that tuple are all visible and that an update to that tuple locks the entire tuple and that set of ints, etc. If the data isn't changed after being loaded and you're able to load an entire tuple all at once then this could work. Note that arrays aren't more efficient than just using individual columns, and this technique is only going to be helpful if the tuple overhead in your situation is a large portion of the data and using this technique allows you to reduce the number of tuples stored. Thanks! Stephen
Attachment:
signature.asc
Description: Digital signature
