Any moron can learn to trudge through the basic steps. There is no recognizable transition, and that is what pisses Randy off, and has always pissed him off, about dancing lessons. Randy knows that the steps they are doing are nominally the same as the basic steps demonstrated earlier, but he's damned if he can tell which is which, once they go into their creative mode. This is, certainly, great, but how can we arrive at it, when there is nothing on the screen but a blinking cursor? «At this point in the videotape he always wonders if he's inadvertently set his beer down on the fast-forward button, or something, because the dancers go straight from their vicious Randy parody into something that obviously qualifies as advanced dancing. WHERE r.departure_airport = p.last_arrival (p.hops || r.arrival_airport)::char(3),īool_or(r.arrival_airport = p.destination) OVER () The query may look somewhat like this: WITH RECURSIVE p(last_arrival, destination, hops, flights, found) AS ( (If you are unaware of the structure of the demo database yet, review its description.) Formally, routes is a view, but we do not need to think about it. PracticeĪrmed with the theory we can now (in theory) write the above query: search of the shortest route from, say, Ust-Kut (UKX) to Nerungri (CNN).įrom the entire demo database, we will need two tables: airports and routes. If something is unclear as early as at this point, it's time to read the documentation. SELECT t.n+1, t.factorial*(t.n+1) FROM t WHERE t.n < 5Īnd this is enough to recall the basics. Here is a simple example: demo=# WITH RECURSIVE t(n,factorial) AS ( Using pseudocode, we may represent this as follows: res ← EMPTY If we replace UNION ALL with UNION, duplicate rows will be eliminated at each iteration. In the process, the output of each iteration is placed into a resulting table, which will be available under the same name of t when all the query is complete (3). The recursive term is called so because it can access the output of the previous iteration that is available under the name of t. Then the recursive term (2) is iterated while it returns some rows.
The non-recursive term (1) is performed first.
And here complications arise since a recursive query is not very similar to a usual query and even less to a usual loop.Ī general format of a recursive query is as follows: WITH RECURSIVE t AS (
It's not that it is pretty often necessary, but it sometimes happens to be. The optimizer gives up and withdraws, leaving you alone face to face with the performance.Ī recursive query is just a technique to implement a loop right in SQL. What SQL dislikes is when one jerks it in a loop row by row rather than have the task performed in one statement. The good old relational SQL adequately does the job with unordered sets: there is a whole panoply of capabilities for them both in the language and «under the hood» of the DBMS.
#CAN WE WRITE QUERIES IN DBVISUALIZER HOW TO#
Let's start with recalling the theory (very briefly since all of it is trivial), and then we will discuss what to do if it is unclear how to approach a real-life problem or if it seems to be clear, but the query persistently fails to work fine.įor an exercise, we will use the airlines demo database and try to write a query to find the shortest route from one airport to another. We will address a more complicated case of an arbitrary graph. This topic was brought up routinely, but the discussion was usually limited to simple cases related to trees: to descend from a vertex to the leaves and to ascend from a vertex to the root. This article deals with writing recursive queries.