Issue in _bt_getrootheight

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От Ahmed Ibrahim
Тема Issue in _bt_getrootheight
Дата
Msg-id CAHiW8txjW0qHjeTEwNLGKuLgOPkNXF2TE-sozjSMeeD7uTm77A@mail.gmail.com
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Ответы Re: Issue in _bt_getrootheight  (Gurjeet Singh <gurjeet@singh.im>)
Список pgsql-hackers
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Hi everyone,

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 */
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 :)

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