Adaptive tiering for database data of a replica group

One or more non-transitory, computer-readable storage media, storing program instructions that when executed on or across one or more computing devices cause the one or more computing devices to implement a replica group for a database that respectfully implements:

• maintaining, by a plurality of nodes of the database, comprising:
• a plurality of portions stored at an external storage service individually organized according to a first type of index structure;
• and
• a local portion organized according to a second type of index structure different from the first type of index structure;
• and
• electing a leader node of the plurality of nodes of the replica group based at least in part on metrics determined for the individual nodes of the plurality of nodes, wherein the respective metrics individually comprise indications of respective fractions of the database stored in the respective local portions of the respective nodes.

A system, comprising:

• a plurality of nodes, individually comprising at least one processor and a memory, implementing a replica group for a database, the plurality of nodes configured to maintain a database comprising:
• a plurality of portions stored at an external storage service individually organized according to a first type of index structure;
• and
• a local portion organized according to a second type of index structure different from the first type of index structure;
• and
• a control plane, comprising at least one processor and a memory, for the replica group, configured to elect a leader node of the plurality of nodes of the replica group based at least in part on metrics determined for the individual nodes of the plurality of nodes, wherein the respective metrics individually comprise indications of respective fractions of the database stored in the respective local portions of the respective nodes.