How to Correctly Return SELECT Query Results from a PostgreSQL Function?

Return SELECT query results in PostgreSQL function

In PostgreSQL, using functions to return the results of a SELECT query requires understanding the appropriate syntax and return type. We'll examine one of the provided functions and guide you on how to correctly retrieve query results.

The function in question wordFrequency currently lacks a correct return statement. To solve this problem, we will use the RETURN QUERY syntax.

Corrected function:

<code class="language-sql">CREATE OR REPLACE FUNCTION wordFrequency(_max_tokens integer)
RETURNS SETOF RECORD AS $$
BEGIN
  RETURN QUERY
  SELECT text, count(*), 100.0 / _max_tokens * count(*) AS ratio
  FROM (
    SELECT text
    FROM token
    WHERE chartype = 'ALPHABETIC'
    LIMIT _max_tokens
  ) AS tokens
  GROUP BY text
  ORDER BY count DESC;
END;
$$ LANGUAGE plpgsql;</code>

Key points:

The
• RETURN QUERY syntax allows you to directly return the results of a SELECT query.
• SETOF RECORD indicates that the function will return a set of records.
• Output parameters in the function body (such as text and count(*)) must match the column names in the query. We added AS ratio to give the third column an ​​unambiguous name and avoid ambiguity. At the same time, change 100 to 100.0 to avoid precision loss caused by integer division.

Call function:

<code class="language-sql">SELECT * FROM wordFrequency(123);</code>

Additionally, using RETURNS TABLE to explicitly define the return type is more practical than returning a generic record because it eliminates the need to specify a list of column definitions for every function call. For example:

<code class="language-sql">CREATE OR REPLACE FUNCTION wordFrequency(_max_tokens integer)
RETURNS TABLE (text TEXT, cnt BIGINT, ratio NUMERIC) AS $$
BEGIN
  RETURN QUERY
  SELECT text, count(*), 100.0 / _max_tokens * count(*)
  FROM (
    SELECT text
    FROM token
    WHERE chartype = 'ALPHABETIC'
    LIMIT _max_tokens
  ) AS tokens
  GROUP BY text
  ORDER BY count DESC;
END;
$$ LANGUAGE plpgsql;</code>

Important Notes:

• Choose output parameter names carefully to avoid conflicts with column names in the query.
• Use the ordinal position of the item in the SELECT list or repeat the expression in ORDER BY to resolve potential naming conflicts.
• Consider using numeric data types or multiplying before dividing in calculations involving integers to minimize rounding errors.

By following these steps, you can efficiently use functions in PostgreSQL to return the results of a SELECT query.

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