sequence

Why does PostgreSQL perform sequential scan on indexed column?

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Very simple example – one table, one index, one query: CREATE TABLE book ( id bigserial NOT NULL, “year” integer, — other columns… ); CREATE INDEX book_year_idx ON book (year) EXPLAIN SELECT * FROM book b WHERE b.year > 2009 gives me: Seq Scan on book b (cost=0.00..25663.80 rows=105425 width=622) Filter: (year > 2009) Why … Read more

Change the name of a key in dictionary

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I want to change the key of an entry in a Python dictionary. Is there a straightforward way to do this? 2Best Answer 21 Easily done in 2 steps: dictionary[new_key] = dictionary[old_key] del dictionary[old_key] Or in Best Answertep: dictionary[new_key] = dictionary.pop(old_key) which will raise KeyError if dictionary[old_key] is undefined. Note that this will delete dictionary[old_key]. … Read more

How can I get the concatenation of two lists in Python without modifying either one? [duplicate]

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This question already has answers here: How do I concatenate two lists in Python? (31 answers) Closed 6 years ago. In Python, the only way I can find to concatenate two lists is list.extend, which modifies the first list. Is there any concatenation function that returns its result without modifying its arguments? 7 s 7 … Read more

Understanding slicing

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I need a good explanation (references are a plus) on Python slicing. 3 34 It’s pretty simple really: a[start:stop] # items start through stop-1 a[start:] # items start through the rest of the array a[:stop] # items from the beginning through stop-1 a[:] # a copy of the whole array There is also the step … Read more