CRDT Disk-space requirements v5.8.1

An important consideration is the overhead associated with CRDT types, particularly the on-disk size.

Operation-based CRDT disk-space reqs

For operation-based types, this is trivial because the types are merely domains on top of other types. They have the same disk space requirements no matter how many nodes are there:

• crdt_delta_counter — Same as bigint (8 bytes)
• crdt_delta_sum — Same as numeric (variable, depending on precision and scale)

There's no dependency on the number of nodes because operation-based CRDT types don't store any per-node information.

State-based CRDT disk-space reqs

For state-based types, the situation is more complicated. All the types are variable length (stored essentially as a bytea column) and consist of a header and a certain amount of per-node information for each node that modified the value.

For the bigint variants, formulas computing approximate size are:

• crdt_gcounter — 32B (header) + N * 12B (per-node)

• crdt_pncounter -—48B (header) + N * 20B (per-node)

  N denotes the number of nodes that modified this value.

For the numeric variants, there's no exact formula because both the header and per-node parts include numeric variable-length values. To give you an idea of how many such values you need to keep:

• crdt_gsum
  • fixed: 20B (header) + N * 4B (per-node)
  • variable: (2 + N) numeric values
• crdt_pnsum
  • fixed: 20B (header) + N * 4B (per-node)
  • variable: (4 + 2 * N) numeric values