MyRocks and Index-Only Scans

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This article is about MyRocks and index-only scans on secondary indexes. It applies to MariaDB's MyRocks, Facebook's MyRocks, and other variants.

Mem-comparable keys

MyRocks indexes store "mem-comparable keys" (that is, the key values are compared with memcmp). For some datatypes, it is easily possible to convert between the column value and its mem-comparable form, while for others the conversion is one-way.

For example, in case-insensitive collations capital and regular letters are considered identical, i.e. 'c' ='C'. For some datatypes, MyRocks stores some extra data which allows it to restore the original value back. (For the latin1_general_ci collation and character 'c', for example, it will store one bit which says whether the original value was a small 'c' or a capital letter 'C'). This doesn't work for all datatypes, though.

Index-only support for various datatypes

Index-only scans are supported for numeric and date/time datatypes. For CHAR and VAR[CHAR], it depends on which collation is used, see below for details)

Index-only scans are currently not supported for less frequently used datatypes, like

  • BIT(n)
  • SET(...)
  • ENUM(...) It is actually possible to add support for those, feel free to write a patch or at least make a case why a particular datatype is important

Index-only support for various collations

As far as Index-only support is concerned, MyRocks distinguishes three kinds of collations:

1. Binary (reversible) collations

These are binary, latin1_bin, and utf8_bin.

For these collations, it is possible to convert a value back from its mem-comparable form. Hence, one can restore the original value back from its index record, and index-only scans are supported.

2. Restorable collations

These are collations where one can store some extra information which helps to restore the original value.

Criteria (from storage/rocksdb/rdb_datadic.cc, rdb_is_collation_supported())

  • The charset should use 1-byte characters (so, unicode-based collations are not included)
  • strxfrm(1 byte) = {one 1-byte weight value always}
  • no binary sorting
  • PAD attribute

The examples are: latin1_general_ci, latin1_general_cs, latin1_swedish_ci, etc.

Index-only scans are supported for these collations.

3. All other collations

For these collations, there is no known way to restore the value from its mem-comparable form, and so index-only scans are not supported.

MyRocks needs to fetch the clustered PK record to get the field value.

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