Hello,
> ---------------------------------------------------------------------
> -------------------------
> PostgreSQL 9.3.4 on x86_64-unknown-linux-gnu, compiled by gcc
> (Debian
> 4.8.3-2) 4.8.3, 64-bit
>
>
> ---------------------------------------------------------------------
> ------------------------------------------------
> PostgreSQL 15.3 (Debian 15.3-1.pgdg120+1) on x86_64-pc-linux-gnu,
> compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit
> (1 Zeile)
>
>
9.3 plan
-> Index Scan using client_session_user_id_idx on client_session
Looks like a collation issue given the difference in compilers used.
In the 9.3 plan the index is used.
Maybe try a reindex of the table.
HTH,
Rob