Re: Issue in _bt_getrootheight

On Tue, Jul 11, 2023 at 9:35 AM Ahmed Ibrahim
<ahmed.ibr.hashim@gmail.com> wrote:
>
> We have been working on the pg_adviser extension whose goal is to suggest indexes by creating virtual/hypothetical
indexesand 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
calculatethe 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
thatin 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
calculateit 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 think the better method would be to calculate the index height
*after* get_relation_info_hook is called. That way, instead of the
server guessing whether or not an index is hypothetical it can rely on
the index adviser's notion of which index is hypothetical. The hook
implementer has the opportunity to not only mark the
indexOptInfo->hypothetical = true, but also calculate the tree height,
if they can.

Please see attached the patch that does this. Let me know if this patch helps.

Best regards,
Gurjeet
http://Gurje.et