manipulating anyarray columns

[Tomas Vondra <tv@xxxxxxxx>]

Hi,

I have to deal with anyarray columns for the first time, and I've run
into some trouble. I need to read the pg_stats.most_common_vals column,
and read several items, i.e. most frequent values stored in the column.

With a regular column (as for example most_common_freqs) I can do this

  SELECT most_common_freqs[1:3]
  FROM pg_stats
  WHERE tablename = 'pg_attribute' AND attname = 'attname';

to get the first 3 items (frequencies), but with anyarray, I can't do
that - for example

  SELECT most_common_vals[1:3]
  FROM pg_stats
  WHERE tablename = 'pg_attribute' AND attname = 'attname';
  ERROR:  cannot subscript type anyarray because it is not an array

I do understand that many operations are not available with anyarray as
the actual type is unknown (it could be an array of arrays etc.) but why
is this impossible?

Surprisingly, I can do this:

  SELECT
    (string_to_array(array_to_string(most_common_vals,','),','))[1:3]
  FROM pg_stats
  WHERE tablename = 'pg_attribute' AND attname = 'attname';

which is good enough for me, but I'm curious why the simple approach
does not work. Or am I missing something / using it incorrectly?

Tomas

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