RethinkDB has a few hard limitations, as well as some soft limitations that are dependent on your server configurations.
There is no hard limit on the number of databases that can be created.
There is a hard limit of 64 shards. (See Sharding and replication for more information.)
There is no hard limit on the number of tables per database or cluster.
Each table requires a minimum of approximately 10MB disk space on each server in a cluster. (A completely empty table takes up 4MB.)
Each table has an overhead of 8MB RAM on each server it’s replicated on.
While there is no hard limit on the size of a single document, there is a recommended limit of 16MB for memory performance reasons.
The maximum size of a JSON query is 64M.
RethinkDB requires data structures in RAM on each server proportional to the size of the data on that server’s disk, usually around 1% of the size of the total data set. See Understanding RethinkDB memory requirements for more details.
Primary keys are limited to 127 characters.
Secondary keys are indexed on their first 238−PK bytes, where PK is the primary key length of that table. If a secondary index has keys whose first 238−PK bytes are identical, performance using those keys will be degraded, as RethinkDB will fall back on a linear search.
Secondary indexes do not store objects or null values. See Using secondary indexes for more details.
Primary key strings may not include the null codepoint (U+0000).
Numbers are double precision IEEE 754 floating point. Integers from −253 to 253 are stored precisely; integers outside that range may be rounded. RethinkDB does not allow NaN or infinite values.
By default, arrays on the RethinkDB server have a size limit of 100,000 elements. This can be changed on a per-query basis with the arrayLimit (or array_limit) option to run.
RethinkDB uses byte-wise ordering for indexes, orderBy and between. While this corresponds to codepoint ordering in UTF-8, RethinkDB does not support Unicode collations, and does not normalize for identical characters with multiple codepoints (i.e, \u0065\u0301 and \u00e9 both represent the character “é” but RethinkDB treats them, and sorts them as, distinct characters).
Some file systems, typically compressed or encrypted ones, do not support the --direct-io option. (See RethinkDB command line options for a full list of supported options.)
There are currently issues with btrfs. Follow issue #2781 for more information.
By default, RethinkDB can return data from concurrent writes that have not been committed to disk yet. The read_mode option to table allows control of the isolation level (starting in RethinkDB 2.1).