Postgres Query Based on Previous and Next Rows - sql

Question

I'm trying to solve the bus routing problem in postgresql which requires visibility of previous and next rows. Here is my solution.
Step 1) Have one edges table which represents all the edges (the source and target represent vertices (bus stops):
postgres=# select id, source, target, cost from busedges;
id | source | target | cost
----+--------+--------+------
1 | 1 | 2 | 1
2 | 2 | 3 | 1
3 | 3 | 4 | 1
4 | 4 | 5 | 1
5 | 1 | 7 | 1
6 | 7 | 8 | 1
7 | 1 | 6 | 1
8 | 6 | 8 | 1
9 | 9 | 10 | 1
10 | 10 | 11 | 1
11 | 11 | 12 | 1
12 | 12 | 13 | 1
13 | 9 | 15 | 1
14 | 15 | 16 | 1
15 | 9 | 14 | 1
16 | 14 | 16 | 1
Step 2) Have a table which represents bus details like from time, to time, edge etc.
NOTE: I have used integer format for "from" and "to" column for faster results as I can do an integer query, but I can replace it with any better format if available.
postgres=# select id, "busedgeId", "busId", "from", "to" from busedgetimes;
id | busedgeId | busId | from | to
----+-----------+-------+-------+-------
18 | 1 | 1 | 33000 | 33300
19 | 2 | 1 | 33300 | 33600
20 | 3 | 2 | 33900 | 34200
21 | 4 | 2 | 34200 | 34800
22 | 1 | 3 | 36000 | 36300
23 | 2 | 3 | 36600 | 37200
24 | 3 | 4 | 38400 | 38700
25 | 4 | 4 | 38700 | 39540
Step 3) Use dijkstra algorithm to find the nearest path.
Step 4) Get the upcoming buses from the busedgetimes table in the earliest first order for the nearest path detected by dijkstra algorithm.
Problem: I am finding it difficult to make the query for the Step 4.
For example: If I get the path as edges 2, 3, 4, to travel from source vertex 2 to target vertex 5 in the above records. To get the first bus for the first edge, it's not so hard as I can simply query with from < 'expected departure' order by from desc but for the second edge, the from condition requires to time of first result row. Also, query requires edge ids filter.
How can I achieve this in a single query?

Answer 1

I am not sure if I understood your problem correctly. But getting values from other rows this can be done by window functions (https://www.postgresql.org/docs/current/static/tutorial-window.html):
demo: db<>fiddle
SELECT
id,
lag("to") OVER (ORDER BY id) as prev_to,
"from",
"to",
lead("from") OVER (ORDER BY id) as next_from
FROM bustimes;
The lag function moves the value of the previous row into the current one. The lead function does the same with the next row. So you are able to calculate a difference between last arrival and current departure or something like that.
Result:
id prev_to from to next_from
18 33000 33300 33300
19 33300 33300 33600 33900
20 33600 33900 34200 34200
21 34200 34200 34800 36000
22 34800 36000 36300
Please notice that "from" and "to" are reserved words in PostgreSQL. It would be better to chose other names.