I am trying to understand the query plan of MonetDB.
Is there a documentation anywhere where I can find what each instruction stays for? If not, can anybody tell me what are returning
sql.projectdelta(X_15,X_23,X_25,r1_30,X_27)
and
sql.subdelta(X_246,X_4,X_10,X_247,X_249), for example?
In my query I am sorting the result by two attributes (e.g., by A,B). Can you tell me why the second sort has more parameters than the first?
(X_29,r1_36,r2_36) := algebra.subsort(X_28,false,false);
(X_33,r1_40,r2_40) := algebra.subsort(X_22,r1_36,r2_36,false,false);
Is algebra.subsort returning (oid, columnType) pairs, or just oid?
Thank you!!
Understanding output of the explain SQL statement requires knowledge of the MonetDB Assembly-like Language (MAL).
Concerning functions sql.projectdelta, sql.subdelta, and algebra.subsort, you'll find their signature and a (brief) description in the monetdb lib folder. Ex :
Concerning the different number of parameters for algebra.subsort :
(X_29,r1_36,r2_36) := algebra.subsort(X_28,false,false); is described as :
Returns a copy of the BAT sorted on tail values, a BAT that specifies how the input was reordered, and a BAT with group information. The input and output are (must be) dense headed. The order is descending if the reverse bit is set. This is a stable sort if the stable bit is set.
(X_33,r1_40,r2_40) := algebra.subsort(X_22,r1_36,r2_36,false,false); is described as:
Returns a copy of the BAT sorted on tail values, a BAT that specifies how the input was reordered, and a BAT with group information. The input and output are (must be) dense headed. The order is descending if the reverse bit is set. This is a stable sort if the stable bit is set.
MAL functions can be overloaded bassed on their return value. algebra.subsort can return 1, 2 or 3 values depending on what you're asking for. Checl algebra.mal for the different possibilities.