On Thu, Aug 29, 2024 at 03:48:53PM -0500, Masahiko Sawada wrote: > On Sun, May 19, 2024 at 6:46 AM Noah Misch <n...@leadboat.com> wrote: > > If I were standardizing pg_trgm on one or the other notion of "char", I > > would > > choose signed char, since I think it's still the majority. More broadly, I > > see these options to fix pg_trgm: > > > > 1. Change to signed char. Every arm64 system needs to scan pg_trgm indexes. > > 2. Change to unsigned char. Every x86 system needs to scan pg_trgm indexes. > > Even though it's true that signed char systems are the majority, it > would not be acceptable to force the need to scan pg_trgm indexes on > unsigned char systems. > > > 3. Offer both, as an upgrade path. For example, pg_trgm could have separate > > operator classes gin_trgm_ops and gin_trgm_ops_unsigned. Running > > pg_upgrade on an unsigned-char system would automatically map v17 > > gin_trgm_ops to v18 gin_trgm_ops_unsigned. This avoids penalizing any > > architecture with upgrade-time scans. > > Very interesting idea. How can new v18 users use the correct operator > class? I don't want to require users to specify the correct signed or > unsigned operator classes when creating a GIN index. Maybe we need to
In brief, it wouldn't matter which operator class new v18 indexes use. The documentation would focus on gin_trgm_ops and also say something like: There's an additional operator class, gin_trgm_ops_unsigned. It behaves exactly like gin_trgm_ops, but it uses a deprecated on-disk representation. Use gin_trgm_ops in new indexes, but there's no disadvantage from continuing to use gin_trgm_ops_unsigned. Before PostgreSQL 18, gin_trgm_ops used a platform-dependent representation. pg_upgrade automatically uses gin_trgm_ops_unsigned when upgrading from source data that used the deprecated representation. What concerns might users have, then? (Neither operator class would use plain "char" in a context that affects on-disk state. They'll use "signed char" and "unsigned char".) > dynamically use the correct compare function for the same operator > class depending on the char signedness. But is it possible to do it on > the extension (e.g. pg_trgm) side? No, I don't think the extension can do that cleanly. It would need to store the signedness in the index somehow, and GIN doesn't call the opclass at a time facilitating that. That would need help from the core server.
