[erlang-patches] New index type in mnesia (new feature)

Fredrik <>
Fri Jun 28 12:22:55 CEST 2013


On 06/19/2013 06:51 PM, Aleksander Nycz wrote:
> Hello,
>
> Mnesia gives possibility to create table indexes, when
> the user wants to frequently use some other field
> than the key field to look up records.
>
> Current index solution in mnesia uses ets table (type bag or 
> duplicated_bag) to maintain mapping:
> Indexed field value -> Primary key value.
>
> Unfortunatelly current solution has very significant disadvantage:
> operation performance (loading table, insert new records,
> delete records, etc.) is very low when index is set on 
> 'Low-cardinality column'
>
> http://en.wikipedia.org/wiki/Cardinality_%28SQL_statements%29
>
> In such case operation complexity is O(n) when n is number
> of Primary Key Values. For small n performance can be acceptable for 
> some application,
> but when n is the hundreds, thousands or even more such index
> are useless. New index type provides O(1) complexity.
>
> This patch introduces new index type in mnesia database.
> Main concept is to maintain all Primary Key Values not direcly in
> bag/duplicated_bag ets but in set of ets.
> For each Indexed field value new ets is created
> and Primary Key Values are strored in this ets.
> For 'Low-cardinality column' there is only a few Indexed key value 
> (eg. isActive (true/false), state (new/pending/suspended/active), ...)
> so memory overhead for ets is not significant.
>
> Standard index:
>     Indexed field value -> [Primary key value]
>
> New index based on ets:
>     Indexed field value -> ets, that contains Primary key value
>
> Restrictions:
>
> 1. New index can be created on disc_copies or ram_copies tables only. 
> Tables disc_only_copies are not supported.
> 2. Index type can't be changed. The only way to change existing index 
> idx_list to idx_ets and vice versa
>      is to delete existing index and create new one by 
> mnesia:add_table_index/3 (new function, see below)
>
>
> New API:
>
> 1. Define index type when table is created:
>
> create_table(Name, TabDef) -> {atomic, ok} | {aborted, Reason}
>
> New TabDef value:
> {index_type, [{atom() | int(), 'idx_std' | 'idx_ets'}]} - 'idx_std' is 
> default when index is created
>
> Example:
>
> -type(poolId() :: integer()).
> -type(bucketId() :: integer()).
> -type(resourceState() :: free | reserved | gracePeriod).
>
> -record(rmResource, {id                                 :: {poolId(), 
> any()}
>                     ,state                              :: {poolId(), 
> bucketId(), resourceState()}
>                     ,availableFrom                      :: integer()
>                     ,availableTo                        :: integer()
>                     ,requestorId                        :: any()
>                     ,reservedFrom                       :: integer()
>                     ,reservedTo                         :: integer()
>                     ,isDeleted      = false             :: boolean()
>                     ,mTime                              :: integer()}).
>
>      {atomic,ok} = mnesia:create_table(tRMResources
>                                       ,[
>                                          {disc_copies, []}
>                                         ,{ram_copies, [node()]}
>                                         ,{type,set}
> ,{attributes,record_info(fields, rmResource)}
>                                         ,{record_name, rmResource}
>                                         ,{index, [state, requestorId, 
> mTime]}
>                                         ,{index_type, [{state, 
> idx_ets}, {requestorId, idx_std}]}
>                                        ]),
>
> 2. Add new index to existing table:
>
> mnesia:add_table_index(Tab, AttrName, IndexOpts) -> {aborted, R} | 
> {atomic, ok}
>
> This function creates a index on Mnesia table called Tab on AttrName
> field according to the argument IndexOpts.
> This list must be a list of {Item, Value} tuples, currently only one
> option is allowed:
>      {index_type, 'idx_std' | 'idx_ets'}
>
> Example:
>
> mnesia:add_table_index(tRMResources, isDeleted, [{index_type, 
> 'idx_ets'}])
>
> 3. New match_object/4, dirty_match_object/3 functions:
>
> match_object(Tab, Pat, Limit, LockKind) -> [Record] | transaction abort.
> dirty_match_object(Tab, Pat, Limit) -> [Record] | exit({aborted, 
> Reason}).
>
> Similar to match_object/3 and dirty_match_object/2, but returns no 
> more than Limit records.
>
>
> 4. New index_match_object/5, dirty_index_match_object/4 functions:
>
> index_match_object(Tab, Pat, Attr, Limit, LockKind) -> [Record] | 
> transaction abort.
> dirty_index_match_object(Tab, Pat, Attr, Limit) -> [Record] | 
> exit({aborted, Reason}).
>
> Similar to index_match_object/4, dirty_index_match_object/3 but 
> returns no more than Limit records.
>
>
> 5. New index_read/4, dirty_index_read/4 functions:
>
> index_read(Tab, Key, Attr, Limit) -> [Record] | transaction abort.
> dirty_index_read(Tab, Key, Attr, Limit) -> [Record] | exit({aborted, 
> Reason}).
>
> Similar to index_read/3, dirty_index_read/3 but returns no more than 
> Limit records.
>
>
> 6. New select_limit/3, select_limit/4, dirty_select/3 functions;
>
> select_limit(Tab, MatchSpec, NObjects [, Lock]) -> [Object] | 
> transaction abort.
>
> Similar to select(Tab, MatchSpec [, Lock]) but returns maximum NObjects
> records, of course empty list can also be returned.
> Continuation (see select/4) is not possible. This function can also use
> indexes to find matching records
> as contrasted with select/4.
>
> dirty_select(Tab, Spec, Limit) -> [Object] | exit({aborted, Reason}.
>
> Similar to dirty_select/2 but returns no more than Limit records.
>
> And git links:
>
> git fetch git://github.com/nyczol/otp.git mnesia_new_index
>
> https://github.com/nyczol/otp/compare/erlang:master...mnesia_new_index
> https://github.com/nyczol/otp/compare/erlang:master...mnesia_new_index.patch 
>
>
> Regards,
> Aleksander Nycz
>
>
>
> _______________________________________________
> erlang-patches mailing list
> 
> http://erlang.org/mailman/listinfo/erlang-patches
Hello,
Please rebase this patch upon current maint. No mergeing ;)
Thanks,

-- 

BR Fredrik Gustafsson
Erlang OTP Team

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