We have been working on the pg_adviser extension whose goal is to suggest indexes by creating virtual/hypothetical indexes and see how it affects the query cost.
The hypothetical index shouldn't take any space on the disk (allocates 0 pages) so we give it the flag INDEX_CREATE_SKIP_BUILD.
But the problem comes from here when the function get_relation_info is called in planning stage, it tries to calculate the B-Tree height by calling function _bt_getrootheight, but the B-Tree is not built at all, and its metadata page (which is block 0 in our case) doesn't exist, so this returns error that it cannot read the page (since it doesn't exist).
I tried to debug the code and found that this feature was introduced in version 9.3 under this commit [1]. I think that in the code we need to check if it's a B-Tree index AND the index is built/have some pages, then we can go and calculate it otherwise just put it to -1
I mean instead of this
if (info->relam == BTREE_AM_OID)
{
/* For btrees, get tree height while we have the index open */