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GIN and GiST indexes utilizing pg_trgm's opclasses store sorted trigrams within index tuples. When comparing and sorting each trigram, pg_trgm treats each character as a 'char[3]' type in C. However, the char type in C can be interpreted as either signed char or unsigned char, depending on the platform, if the signedness is not explicitly specified. Consequently, during replication between different CPU architectures, there was an issue where index scans on standby servers could not locate matching index tuples due to the differing treatment of character signedness. This change introduces comparison functions for trgm that explicitly handle signed char and unsigned char. The appropriate comparison function will be dynamically selected based on the character signedness stored in the control file. Therefore, upgraded clusters can utilize the indexes without rebuilding, provided the cluster upgrade occurs on platforms with the same character signedness as the original cluster initialization. The default char signedness information was introduced in 44fe30fdab6, so no backpatch. Reviewed-by: Noah Misch <noah@leadboat.com> Discussion: https://postgr.es/m/CB11ADBC-0C3F-4FE0-A678-666EE80CBB07%40amazon.com