[odb-users] Nested vector

[Boris Kolpackov]

Hi,

Zeyang Tao <zeyangtao1020 at gmail.com> writes:

> Recently we had a slowness issue in ODB, the thing is ODB doesn't support
> nested vector, i.e vector of vector, so in order to store that kind of data
> structure, we have to use the following hacky way,
> 
> struct node{
> int x, y;
> double value;
> }
> 
> class parameter{
> ...
> ...
> vector<node> cnt;
> }

One thing you can improve on here is to use the vector's index for 'x',
e.g.,

struct node
{
  size_t y;
  double value;
};

vector<node> cnt;

Or, if you want to keep x and y, you can mark the cnt member as
unordered (Section 14.4.19).


> basically the cnt is a matrix and we use x and y as the index, each single
> node will be a record in the sub table, sometimes the matrix is large and
> one update will write N^2 records to the subtable, plus this update is very
> frequent, so we saw 2-3 seconds delay constantly

I think the first thing you should try is the change-tracking vector 
(Section 5.4). Unless you always modify most of your elements, I bet
it will solve your problem.

Also, there could be some database system-specific ways to optimize
this (e.g., some databases support arrays of simple types). But since
you didn't mention which database you are using (always are good idea),
I can't recommend anything specific in this area.

Boris