How to extract hour from query in postgres

Straightforward vs. convoluted queries

Query complexity may look impressive but often it's completely unnecessary. When using EXTRACT, there is no need for data type flipping gymnastics. Keep it plain, simple, and sleek:

-- The suave secret agent way
SELECT EXTRACT(HOUR FROM observation_time) FROM smartvakt_device_report;

-- The rookie way with fanciful conversions (unnecessary and messier)
SELECT EXTRACT(HOUR FROM CAST(observation_time AS VARCHAR)) FROM smartvakt_device_report;
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Core ways to extract hours

to_char for String Needs

When you're looking to return the hour as a string instead of an integer, the to_char function is your hero:

SELECT to_char(observation_time, 'HH24') AS hour_string FROM your_table;
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date_part – The Stylish Twin

The date_part function can be a slick alternative, especially when you want to keep your code's aesthetic consistent:

SELECT date_part('hour', observation_time) AS hour FROM your_table;
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Keep an eye on your syntax, mate. Make sure the column name matches that fancy table schema of yours. Regular quotes or backticks only complicate things around column names.

The devil is in the details

Timezone-Savvy Timestamps

When dealing with timezone-informed timestamps, the EXTRACT function operates on local time. So, if you need to fetch the hour for 'UTC', for example, we need a conversion in place:

-- Your timezone ninja move here
SELECT EXTRACT(HOUR FROM observation_time AT TIME ZONE 'UTC') FROM your_table;
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Daylight Saving Time Shenanigans

Daylight Saving Time can mess with time extraction. If your timestamp range includes these changes, apply adjustments accordingly. It's like real-life time travel; things get weird!

Large Data and Indexes

For voluminous datasets, the role of indexes on timestamp columns is critical. Functions like EXTRACT might not always make full use of these indexes; remember this when assessing query performance.