How to select timestamp values in PostgreSQL under conditions?

You can try to use row_number window function to make row number which is the closest row from table1 by begin.

then use coalesce function to let begin time is earlier than any of the p_time then the p_time value in the combined table would be the same as begin time and the percent value would be 50

PostgreSQL 9.6 Schema Setup:

CREATE TABLE table1(
  f_key INT,
  begin DATE,
  counts INT
);

INSERT INTO table1 VALUES (1,'2018-10-04',15);
INSERT INTO table1 VALUES (1,'2018-10-06',20);
INSERT INTO table1 VALUES (1,'2018-10-08',34);
INSERT INTO table1 VALUES (1,'2018-10-09',56);

CREATE TABLE table2(
  f_key INT,
  p_time DATE,
  percent INT
);


INSERT INTO table2 VALUES (1, '2018-10-05',80);
INSERT INTO table2 VALUES (1, '2018-10-07',90);
INSERT INTO table2 VALUES (1, '2018-10-08',70);
INSERT INTO table2 VALUES (1, '2018-10-10',60);

Query 1:

SELECT ROW_NUMBER() OVER(ORDER BY begin) "row",
       t1.f_key,
       t1.counts,
       coalesce(t1.p_time,t1.begin) p_time,
       coalesce(t1.percent,50) percent
FROM (
   SELECT ROW_NUMBER() OVER(PARTITION BY t1.begin,t1.f_key order by t2.p_time desc) rn,
       t2.p_time,
       t2.percent,
       t1.counts,
       t1.f_key,
       t1.begin
   FROM table1 t1 
   LEFT JOIN table2 t2 ON t1.f_key = t2.f_key and t1.begin > t2.p_time
)t1
WHERE rn = 1

Results:

| row | f_key | counts |     p_time | percent |
|-----|-------|--------|------------|---------|
|   1 |     1 |     15 | 2018-10-04 |      50 |
|   2 |     1 |     20 | 2018-10-05 |      80 |
|   3 |     1 |     34 | 2018-10-07 |      90 |
|   4 |     1 |     56 | 2018-10-08 |      70 |