descpb.TableDescriptor structs are 1104 bytes large (ignoring internal references). tabledesc.immutable structs embed a descpb.TableDescriptor and are 1168 byte large. Due to this size, we should strive to minimize the number of times that we copy, re-allocate, or serialize these structs on the hot path of query processing.

@ajwerner, a man ahead of his time, has TODOs that allude to this:

In recent experimentation, I've observed these copies in CPU and heap profiles. On a 100% read workload, immutable allocations are not only the second largest source of heap allocations (by size), but they also appear prominently in CPU profiles. For instance, in kv100 (which is CPU-bound), TableReaderSpec.BuildTableDescriptor is responsible for 1.33% of all CPU utilization.

Jira issue: CRDB-12042