Table Pushdown Optimizer Rule
openLooKeng supports Table Pushdown optimizer rule to improve latency for eligible queries.
The user must execute the following command to generate stats for all the tables which are part of the SQL query so that the join sources are reordered based on the size of the tables-
ANALYZE tableName;
User Hint Comment-
The user may also add a special comment in the following format to leverage the benefit from the Table Pushdown rule by specifying the table names and their distinct(primary key) column names.
/* #distinct@ table1 = col1, col2, ... #*/
This hint is optional as openLooKeng can identify all distinct columns if the relevant statistics are available for the table.
The following query (modified from Query 17 of TPC-H benchmark) is one such eligible query and pushes the outer table into the subquery and improves overall query latency.
Original Query-
SELECT
Sum(lineitem.extendedprice) / 7.0 AS avg_yearly
FROM
lineitem,
part,
(
SELECT
0.2 * Avg(lineitem.quantity) AS s_avg, lineitem.partkey AS s_partkey
FROM
lineitem
GROUP BY
lineitem.partkey
)
WHERE
part.partkey = lineitem.partkey
AND
part.brand = 'Brand#43'
AND
part.container = 'LG PACK'
AND
part.partkey = s_partkey
AND
lineitem.quantity < s_avg /* #distinct@ part = partkey #*/;
In the above query, table part
is the relevant outer query table with partkey
as the unique column, being joined with table lineitem
. An equivalent rewritten query, with part
pushed into the subquery is given as follows.
Equivalent Rewritten Query-
SELECT
Sum(lineitem.extendedprice) / 7.0 AS avg_yearly
FROM
lineitem,
(
SELECT
0.2 * Avg(lineitem.quantity) AS s_avg, lineitem.partkey AS s_partkey
FROM
lineitem, part
WHERE
part.brand = 'Brand#43'
AND
part.container = 'LG PACK'
AND
part.partkey = lineitem.partkey
GROUP BY
lineitem.partkey
)
WHERE
s_partkey = lineitem.partkey
AND
lineitem.quantity < s_avg;
The feature is disabled by default. The user can enable it by executing the following command to set the session parameter-
SET SESSION push_table_through_subquery = true;