Re: Statistics Import and Export

On Tue, 2024-04-02 at 12:59 -0400, Corey Huinker wrote:
> However, some of the ANYARRAYs have element types that are
> themselves arrays, and near as I can tell, such a construct is not
> expressible in SQL. So, rather than getting an anyarray of an array
> type, you instead get an array of one higher dimension.

Fundamentally, you want to recreate the exact same anyarray values on
the destination system as they existed on the source. There's some
complexity to that on both the export side as well as the import side,
but I believe the problems are solvable.

On the export side, the problem is that the element type (and
dimensionality and maybe hasnull) is an important part of the anyarray
value, but it's not part of the output of anyarray_out(). For new
versions, we can add a scalar function that simply outputs the
information we need. For old versions, we can hack it by parsing the
output of anyarray_send(), which contains the information we need
(binary outputs are under-specified, but I believe they are specified
enough in this case). There may be other hacks to get the information
from the older systems; that's just an idea. To get the actual data,
doing histogram_bounds::text::text[] seems to be enough: that seems to
always give a one-dimensional array with element type "text", even if
the element type is an array. (Note: this means we need the function's
API to also include this extra information about the anyarray values,
so it might be slightly more complex than name/value pairs).

On the import side, the problem is that there may not be an input
function to go from a 1-D array of text to a 1-D array of any element
type we want. For example, there's no input function that will create a
1-D array with element type float4[] (that's because Postgres doesn't
really have arrays-of-arrays, it has multi-dimensional arrays).
Instead, don't use the input function, pass each element of the 1-D
text array to the element type's input function (which may be scalar or
not) and then construct a 1-D array out of that with the appropriate
element type (which may be scalar or not).

Regards,
Jeff Davis