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SHRINK

Syntax​

SHRINK key

Time complexity: O(N) where N is the number of buckets in the hash table before the shrink.

ACL categories: @fast, @write

Compacts the hash table that stores the set or hash at key: Dragonfly rebuilds the table with as many buckets as there are members, rounded up to a power of two and at least 8. For a hash, and for a set when --use_oah_set=false, SHRINK also removes the fields or members whose TTL has passed while it rebuilds the table, and deletes the key if none remain. This command is Dragonfly-specific.

SHRINK applies only to sets and hashes stored as a hash table, which the error message calls the DenseSet encoding. Small hashes stored as a listpack (see --listpack_max_bytes and --listpack_max_field_len), small integer-only sets stored as an intset, and keys of all other types return an error.

The reply depends on the encoding:

  • Sets use the OAH format by default (--use_oah_set), which shrinks the table automatically when members are removed, so SHRINK on a set usually returns 0. It still compacts a set whose members expired and were dropped by a read, because reads do not shrink the table.
  • For hashes, and for sets when --use_oah_set=false, the reply reports how much the bucket array shrank.

Return​

One of the following:

  • Integer reply: the number of bytes by which the bucket array shrank, or 0 if the table was already compact.
  • Nil reply: the key does not exist.
  • Error reply: ERR WRONGTYPE Key is not a set or hash with DenseSet encoding if the key holds another type, or a set or hash in a compact encoding. The error starts with ERR, not WRONGTYPE, so clients that look for WRONGTYPE errors do not recognize it.

Examples​

Create a hash with 1,000 fields, remove all but 10 of them, and shrink it. The number in the reply depends on the size of the table. The second call returns 0 because the table is already compact.

dragonfly> EVAL "for i = 1, 1000 do redis.call('HSET', KEYS[1], 'field' .. i, 'v') end" 1 myhash
(nil)
dragonfly> EVAL "for i = 11, 1000 do redis.call('HDEL', KEYS[1], 'field' .. i) end" 1 myhash
(nil)
dragonfly> HLEN myhash
(integer) 10
dragonfly> SHRINK myhash
(integer) 8064
dragonfly> SHRINK myhash
(integer) 0

The same steps on a set return 0, because removing the members already shrank its table:

dragonfly> EVAL "for i = 1, 1000 do redis.call('SADD', KEYS[1], 'member' .. i) end" 1 myset
(nil)
dragonfly> EVAL "for i = 11, 1000 do redis.call('SREM', KEYS[1], 'member' .. i) end" 1 myset
(nil)
dragonfly> SCARD myset
(integer) 10
dragonfly> SHRINK myset
(integer) 0

A small hash and a missing key:

dragonfly> HSET small field1 a field2 b
(integer) 2
dragonfly> SHRINK small
(error) ERR WRONGTYPE Key is not a set or hash with DenseSet encoding
dragonfly> SHRINK nosuchkey
(nil)