The SQL compiler constructs permutations of tables in the FROM clause of a query. The qualified inner join (x inner join y on p syntax) does not dictate a join order but the outer join syntax does. Otherwise join order is subject to change by the compiler's decision. This can be disabled with the OPTION (ORDER) global query option, see sections below.
The first order tried by the optimizer is the initial left to right order of the FROM clause. The compiler remembers its best result so far attained in trying various compilations, so it will not explore branches of the table permutation tree which, while incomplete, exceed the cost of the best complete result so far. Thus a good guess of initial table order by the programmer may save compilation time.
The compiler will evaluate all expressions and predicates as early as possible.
Consider the query:
select count (*) from item where i_price > (select avg (i_price) from item);
The compiler will detect that the sub query is data-independent from the main from and thus will compute the average price before starting the select for the count. Thus the above query executes in time linear to the item table count instead of quadratic time, as it would without the removal of the invariant.
Views are initially opened into equivalent derived tables. If a derived table in a FROM clause is sufficiently simple, i.e. has no distinct, top, group by, and is not a multiple query expression such as a union, it can be in-lined.
Example 9.44. Derived Tables & Views
create view i as select * from item; select i_id from i;
select i_id from (select * from item) xx;
select i_id from item;
In joins involving views representing joins, the in-lining of a derived table will add degrees of freedom to join order selection, since the tables joined in the view will not have to be laid out contiguously.
When there is a derived table or view expansion which cannot be in-lined, i.e. it has a group by or is a union or such, predicates of the enclosing WHERE phrase are migrated into the derived table itself. The set of predicates that may be thus migrated will be a function of the join order, thus different combinations will be tried.
Example 9.45. Example of Migrating Predicates
select i_id from (select * from item1 union all select * from item2) f where i_id = 11;
select i_id from (select i_id from item1 where i_id = 11 union all select i_id from item2 where i_id = 11) f;
It may happen as a result of view expansion that columns get introduced into derived table selection which are nowhere referenced in the actual query. The compiler will detect this and will not compile code accessing these. (see above for example)
It is possible that view expansions introduce predicates that are never simultaneously true with predicates of the enclosing query. This is recognized and can result in an empty query being produced or in union terms being dropped.
The rules for combining predicates are as follows:
- a < b and a < c -> a < min (a, b)
And similarly for <=, >, and >=.
a in (constant1) and a in (constant2)
becomes identically false if the intersection of the constant lists is empty.
- a = b and a = c
becomes identically false if b and c are different constants.
This can lead to transformations such as
select * from (select * from item where i_type = 2 union all select * from item where i_type = 3) xx where i_type = 2;
select * from (select * from item where i_type = 2 ) xx where i_type = 2;
Once a join order is hypothesized, the compiler picks various indices to use for accessing tables. The predicates applicable to the table in the specific join order being considered, as well as a possible ORDER BY clause affect the index choice. If there is an ORDER BY and there is an index which can be used to directly satisfy this, the compiler tries this index as well as the index which seems best in the light of the available predicates. Thus having an applicable ORDER BY does not always imply index selection. If there is a restrictive index for row selection and an order by which could be done by following an index for which there are no predicates, the restrictive index plus sorting will be preferred.
The general rules for index selection based on predicates are:
|Primary key is preferred if 2 equally good choices exist.|
|Equality conditions are preferred over all others.|
|Uniqueness of an index is a plus.|
In queries involving distributed data the principal cost factor is often the RPC latency between the participating databases. In a local area network environment with no other load we estimate one RPC to be about 5 times longer than the selection of a single row on primary key from a table of 1 million, not counting disk I/O. In wide area contexts and with effective load the difference is still more marked. Thus when considering a loop join between a local and a remote table, it becomes obvious that the remote table should be the outer loop, unless there are extremely restrictive criteria on the local table. The cost model takes such considerations into account.
The best case of remote statement compilation is when the statement can be passed through as is to the remote. This is first checked for the whole query and subsequently for each sub query or derived table. The pass-through is not possible if:
|The expression involves tables from multiple remotes or local and remote ones.|
|The expression involves predicates or functions or SQL constructs which do not exist on the remote.|
Even if a derived table or sub query can be passed through the normal SQL optimization applies, thus Virtuoso will import predicates and suggest a join order for these cases, so that the query gets rewritten with the suggested join order left to right in the text.
If one join has multiple tables from the same remote but also other tables, the optimizer will attempt to group the co-located tables together.
select * from r1 join t1 on r1.k = t1.k join r2 on r1.k = r2.k;
will preferentially be compiled as
select * from (select r1.k as r1_k, ... from r1, r2 where r1.k = r2.k) xx, t1 where r1_k = t1.k;
Note that above the join order is meant left to right and that the derived table of r1, r2 is passed through as a unit. This becomes the leading loop in the join because fewer RPC's are likely to be needed then due to row prefetch. As for placing function calls, the general rule is to place them where the arguments are, thus passed through to the remote if they are defined there. On the other hand, if these are invariant they are always computed locally. The explain function can be used to see the details of the compilation. If a function is described as being defined on the particular remote database, this information is taken into account when compiling.
Starting with Virtuoso 3.0, hash joins are used where appropriate. A hash join is about three to four times faster for a local table than with a loop join using an exact match of primary key, thus the gain is substantial.
A hash join works by scanning the shorter of two joined tables and making a hash table from the join columns. This is only possible for joins involving equality, but most joins tend to have equalities in practice.
A hash join does not need indices to be defined and the hash table constructed for one hash join may be reused by another if the table is not modified in-between.
A hash join will be preferred if there is a sufficient number of estimated accesses to the joined table in the query. If a table is expected to be accessed only once, there is less point in making the temporary hash table.
The hash join temporary structures are disk based and reside in the temporary side of the database. They do not survive a database restart but will be ad hoc reused if such are left over from earlier queries and there is no change in the base table between the construction of the hash and its reuse.
A hash join may be specially beneficial if a remote table is on the inner loop of a loop join. The n random accesses can then be replaced with a single sequential scan of the remote table.
The selected join type can be seen in the explain output. The WITH keyword can be used to explicitly specify a join type for a table, see query options below.