Commits on May 8, 2023

1. spanconfig: enable range coalescing by default …

   Fixes cockroachdb#81008.
   
   We built the basic infrastructure to coalesce ranges across table
   boundaries back in 22.2 as part of cockroachdb#66063. We've enabled this
   optimization for secondary tenants since then, but hadn't for the system
   tenant because of two primary blockers:
   
   - cockroachdb#93617: SHOW RANGES was broken by coalesced ranges.
   - cockroachdb#84105: APIs to compute sizes for schema objects (used in our UI) was
     broken by coalesced ranges.
   
   In both these cases we baked in assumptions about there being a minimum
   of one-{table,index,partition}-per-range. These blockers didn't apply to
   secondary tenants at the time since they didn't have access to SHOW
   RANGES, nor the UI pages where these schema statistics were displayed.
   
   We've addressed both these blockers in the 23.1 cycle as part of
   bridging the compatibility between secondary tenants and yesteryear's
   system tenant.
   - cockroachdb#93644 revised SHOW RANGES and crdb_internal.ranges{,_no_leases}, both
     internally and its external UX, to accommodate ranges straddling
     table/database boundaries.
   - cockroachdb#96223 re-worked our SpanStats API to work in the face of coalesced
     ranges, addressing cockroachdb#84105.
   
   Release note (general change): CockroachDB would previously use separate
   ranges for each table, index, or partition. This is no longer true --
   it's possible now to have multiple tables, indexes, and partitions to
   get packed into the same range. For users with many such "schema
   objects", this will reduce the total range count in their clusters. This
   is especially true if individual tables, indexes, or partitions are
   smaller than the default configured maximum range size (controlled using
   zone configs, specifically the range_max_bytes parameter).
   We made this change to improve scalability with respect to the number of
   schema objects, since the underlying range count now no longer a
   bottleneck. Users upgrading from 22.2, once finalizing their upgrade,
   may observe a round of range merges and snapshot transfers (to power
   said range merges) as a result of this change. If users want to opt-out
   of this optimization, they can use the following cluster setting:
   
     SET CLUSTER SETTING spanconfig.storage_coalesce_adjacent.enabled = false;

   

   irfansharif committed May 8, 2023
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