PostgreSQL中COALESCE适用性与空字符串查询优化的技术咨询

Hey there! Let's break down your questions one by one:

Short answer: COALESCE works with most data types in PostgreSQL, but with a key constraint — all the arguments you pass to it need to be of compatible (or implicitly convertible) data types.

COALESCE is a standard SQL function that returns the first non-NULL value from its arguments. Here's how it behaves across different types:

• String types: Like your example COALESCE(mystr, ''), this converts NULL text values to empty strings.
• Numeric types: You can use it to replace NULL numbers with a default, e.g., COALESCE(price, 0) will return 0 if price is NULL.
• Date/time types: It works here too, e.g., COALESCE(order_date, CURRENT_DATE) falls back to today's date if the order date is NULL.
• Other types: Even boolean, array, or custom types work, as long as all arguments match (or can be converted to) the same type.

The only catch: you can't mix incompatible types (like passing a string and an integer together) — PostgreSQL will throw a type mismatch error if you try that.

First, let's clarify the difference between your approach and Wildplasser's:

• Your query SELECT mystr, mystr1 FROM mytable WHERE mystr = '' only matches empty strings ('').
• Wildplasser's query uses COALESCE(mystr, '') = '' to match both NULL and empty strings (since COALESCE turns NULL into '').

Here are the best options depending on your needs:

1. Only match empty strings:
   Stick with mystr = '' — it's simple, efficient, and makes your intent clear. No need for COALESCE here.

2. Match both NULL and empty strings:

   • The most readable option is mystr IS NULL OR mystr = '' — this does the exact same thing as Wildplasser's COALESCE approach, but is easier for other developers to parse at a glance.
   • If you prefer a more concise (though slightly less readable) version, you can use mystr <@ ARRAY['', NULL], but the OR syntax is generally preferred for clarity.

3. About column_value = ' ':
   This query matches strings that contain a single space character, not empty strings. It's only useful if your business logic specifically looks for values that are a space (rather than truly empty). Unless that's your intent, this isn't the right approach for querying empty or NULL values.

内容的提问来源于stack exchange，提问作者Javi