CASE STATEMENT IN POSTGRESQL GENERATOR
CASE STATEMENT IN POSTGRESQL FREE

For example, consider again the following query that searches a table graph using a link field: The standard method for handling such situations is to compute an array of the already-visited values.

However, often a cycle does not involve output rows that are completely duplicate: it may be necessary to check just one or a few fields to see if the same point has been reached before. Sometimes, using UNION instead of UNION ALL can accomplish this by discarding rows that duplicate previous output rows. When working with recursive queries it is important to be sure that the recursive part of the query will eventually return no tuples, or else the query will loop indefinitely. That column will implicitly be added to the output rows of the CTE. The SEARCH clause specifies whether depth- or breadth first search is wanted, the list of columns to track for sorting, and a column name that will contain the result data that can be used for sorting. This syntax is internally expanded to something similar to the above hand-written forms. ) SEARCH BREADTH FIRST BY id SET ordercol SELECT * FROM search_tree ORDER BY ordercol There is built-in syntax to compute a depth- or breadth-first sort column. The order of the rows within each level is certainly undefined, so some explicit ordering might be desired in any case. However, this is an implementation detail and it is perhaps unsound to rely on it.

The recursive query evaluation algorithm produces its output in breadth-first search order. SELECT t.id, t.link, t.data, path || ROW(t.f1, t.f2) For example, if we needed to track fields f1 and f2: In the general case where more than one field needs to be used to identify a row, use an array of rows. SELECT t.id, t.link, t.data, path || t.id WITH RECURSIVE search_tree(id, link, data, path) AS ( To add depth-first ordering information, you can write this: WITH RECURSIVE search_tree(id, link, data) AS ( For example, consider the following query that searches a table tree using a link field: To create a depth-first order, we compute for each result row an array of rows that we have visited so far. This approach merely provides a convenient way to order the results afterwards. Note that this does not actually control in which order the query evaluation visits the rows that is as always in SQL implementation-dependent. This can be done by computing an ordering column alongside the other data columns and using that to sort the results at the end. It still feels wrong somehow.When computing a tree traversal using a recursive query, you might want to order the results in either depth-first or breadth-first order. One way to do this is, maybe try creating an expression index with function being said case statement? create index fact_sales_case on fact_sales ((case I would do 'explain' with and without case statement and compare query plans. This will prevent any indexes or histogram optimization to be used on that particular column which is important. Ok, so same case statement is appearing in both group by and order by. It's like building a road based on where a driverless truck is going. In which case you're designing your database to suit my other gripe - the query generator.

CASE STATEMENT IN POSTGRESQL GENERATOR

That of course assumes that your query generator can be told to use the mview instead. Then create a view that selects from the materialized view or denormalized table. If a query like this is run often enough, I would suggest that a materialized view be created, or a table with triggers to store company_code and location_code in the same row as the fact information.

CASE STATEMENT IN POSTGRESQL FREE

In PostgreSQL, that means a BitmapAnd combination of several indexes, tricky to accomplish and it wouldn't apply to this query because there's no filtration of the Fact table other than the existence of reference rows in the two dimensions.existence which is guaranteed by the model.so no filtration whatsoever.ĭata Warehouse applications should feel free to denormalize data to suit reporting needs, provided they keep the denormalized data consistent with the rest of the DW (obviously the whole DW is a little behind the authoritative data source, I'm saying the DW just needs to be consistent with itself). Oracle gets around this with 'bitmap join' indexes, but building those are incredibly slow.įact/Dim queries are very heavily reliant on the the query optimizer being able to see the 'star join'. Everything has to go through synthetic keys. Neither of which is the fault of the poster, obviously.įact/Dimension seems almost designed to foil the ability to put useful indexes on the Fact table. This post demonstrates two of my biggest complaints in the RDBMS world: query generators and the Kimball Fact/Dimension model.

Case statement in postgresql