Hi! Here is my proposal on extending the storage engine API to provide a functionality for retrieving random rows from tables (those that have indexes). The storage engines for which I plan to implement this are: MyISAM, Aria, Innodb. Possibly RocksDB, TokuDB. --- a/sql/handler.h +++ b/sql/handler.h @@ -2927,7 +2927,7 @@ class handler :public Sql_alloc /** Length of ref (1-8 or the clustered key length) */ uint ref_length; FT_INFO *ft_handler; - enum init_stat { NONE=0, INDEX, RND }; + enum init_stat { NONE=0, INDEX, RND, RANDOM }; init_stat inited, pre_inited; ........ + virtual int ha_random_sample_init() __attribute__((warn_unused_result)) + { + DBUG_ENTER("ha_random_sample_init"); + inited= RANDOM; + DBUG_RETURN(random_sample_init()); + } + virtual int ha_random_sample(uint inx, + key_range *min_key, + key_range *max_key) + __attribute__((warn_unused_result)) + { + DBUG_ENTER("ha_random_sample"); + DBUG_ASSERT(inited == RANDOM); + DBUG_RETURN(random_sample(inx, min_key, max_key)); + } + virtual int ha_random_sample_end() __attribute__((warn_unused_result)) + { + DBUG_ENTER("ha_random_sample_end"); + inited= NONE; + DBUG_RETURN(random_sample_end()); + } + This is the default implementation for a storage engine which does not support it: + virtual int random_sample_init() { return 0; } ; + virtual int random_sample(uint idx, key_range *min_key, key_range *max_key) + { + return HA_ERR_WRONG_COMMAND; + } + virtual int random_sample_end() { return 0; }; Alternative ideas: random_sample_init() takes the idx as a parameter and random_sample just fetches a row from the range using the index previously specified. The range can be left unspecified with nulls to provide a fetch from the full table range. I don't know enough about storage engine internals to know if an index declaration within the init function instead of within the "sample" function is better. Maybe I am complicating it too much and a simple random_sample() function is sufficient, kind of how ha_records_in_range does it. Thoughts? Vicențiu