Issue in _bt_getrootheight

Hi everyone,

We have been working on the pg_adviser
<https://github.com/DrPostgres/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 */
info->tree_height = _bt_getrootheight(indexRelation);
}
else
{
/* For other index types, just set it to "unknown" for now */
info->tree_height = -1;
}

The first line should be
if (info->relam == BTREE_AM_OID && info->pages > 0)
or use the storage manager (smgr) to know if the first block exists.

I would appreciate it if anyone can agree/approve or deny so that I know if
anything I am missing :)

Thanks everyone :)

[1]
https://github.com/postgres/postgres/commit/31f38f28b00cbe2b9267205359e3cf7bafa1cb97