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