Hi, Aleksey! On Jan 13, Aleksey Midenkov wrote: > > > > But always using expr_arena doesn't necessarily seem to be a correct > > choice. E.g. Arg_comparator creates Item_cache_temporal on every > > fix_field. If vcol expr is re-fixed all the time, expr_arena will > > grow continuously. Same for charset conversion, CASE, IN, etc. I > > suspect that if the vcol expr is always re-fixed, it has to use stmt > > arena. > > stmt_arena cannot be used as long as 'expr' item is created on > expr_arena. 'expr' item is created at the vcol expression parse and is > reused during the whole TABLE lifetime. If some of the containee items > later were allocated on stmt_arena, 'expr' item will try to access > them in a different statement when they are already released. That is > what this bugfix tries to address. > > Yes, this fix introduces a small and in most cases imperceptible > memory leak which can be easily overcame by FLUSH TABLES. The fix for > this leak is non-trivial. > > 1. vcol expr is not always refixed, but conditionally controlled by > vcols_need_refixing. And we must remove this control and truly refix > always. > > 2. we must clone 'expr' item into statement mem_root at each > statement. > > I have attached the leak fix PoC into this email. I do not know > whether this is worth it.
vcols_need_refixing isn't run-time conditional, it depends on vcol flags, a specific vcol always needs refixing or never does. And a table has vcols_need_refixing if any of the vcols nees it. Thus, I think, a simpler fix would be to split the cleanup and fix_fields. Now both are done in fix_session_vcol_expr(). We can do cleanup at the end of every statement and fix_fields at the beginning of a statement. Just like for normal non-persistent items. For every vcol that needs it. This way they can safely use execution arena. Regards, Sergei VP of MariaDB Server Engineering and [email protected] _______________________________________________ Mailing list: https://launchpad.net/~maria-developers Post to : [email protected] Unsubscribe : https://launchpad.net/~maria-developers More help : https://help.launchpad.net/ListHelp
