How to achieve CTE functionality in MySQL 5 .7?

问题

I have a USERSEARCH table that should be used for fast substring searches for users. This feature is for an autocomplete search that occurs while someone is typing in a username or name. However, the query I am interested in will only show matches from users the subset of users the searcher follows. This is found in the USERRELATIONSHIP table.

USERSEARCH
-----------------------------------------------
user_id(FK)    username_ngram          name_ngram
1              "AleBoy leBoy eBoy..."  "Ale le e"
2              "craze123 raze123 ..."  "Craze raze aze ze e"
3              "john1990 ohn1990 ..."  "John ohn hn n"
4              "JJ_1 J_1 _1 1"         "JJ"


USERRELATIONSHIP
-----------------------------------------------
user_id(FK)    follows_id(FK)
2              1
2              3

A query like this would be made when someone has just typed in "Al" (not accounting for user relationships):

SELECT * FROM myapp.usersearch where username_ngram like 'Al%'
        UNION DISTINCT
        SELECT * FROM myapp.usersearch where name_ngram like 'Al%'
        UNION DISTINCT
        SELECT * FROM myapp.usersearch                            
        WHERE MATCH (username_ngram, name_ngram) AGAINST ('Al')  
        LIMIT 10

This is blazingly fast because of the existing indices on username_ngram, name_ngram and FULLTEXT(username_ngram, name_ngram). However, in my application context, I need to restrict the search to users that the searcher is following. I would like to replace the "myapp.usersearch" table with a subset of the "myapp.usersearch" table including only users the searcher is following. Here is what I attempted:

    WITH

--Part 1, restrict the USERSEARCH table to just the users that are followed by searcher

        tempUserSearch AS (SELECT T1.id, T2.username_ngram, T2.name_ngram FROM
        (SELECT follows_id FROM myapp.userrelationship WHERE user_id = {user_idOfSearcher} ) AS T1 
        LEFT JOIN myapp.usersearch AS T2  ON T2.user_id = T1.follows_id)

            SELECT * FROM tempUserSearch where username_ngram like 'Al%'
            UNION DISTINCT
            SELECT * FROM tempUserSearch where name_ngram like 'Al%'
            UNION DISTINCT
            SELECT * FROM tempUserSearch                            
            WHERE MATCH (username_ngram, name_ngram) AGAINST ('Al')  
            LIMIT 10

Unfortunately MySQL 5.7 does not support the CTE WITH clause.

Is there any way to reference part 1 of the query in all subsequent subqueries without re-requerying the user_ids of the users the person follows? (in MySQL 5.7)

Update:

Is there really no way to reference a query multiple times in MySQL 5.7? Something seems off as this appears to me as a fundamental task for any db.

Why not do : "x join y on a or b or c"? The speed of my substring query depends on the following indices:

index(username_ngram)

index(name_ngram)

FULLTEXT(username_ngram, name_ngram)

And using OR is not helped by any indices.

回答1:

MySQL 5.7 does not support common table expression; the WITH syntax is available in version 8.0 only.

Since your existing query runs fast, filtering in an outer query might be viable solution:

SELECT ur.id, ng.username_ngram, ng.name_ngram
FROM myapp.userrelationship ur
INNER JOIN (
    SELECT * FROM myapp.usersearch WHERE username_ngram LIKE 'Al%'
    UNION DISTINCT
    SELECT * FROM myapp.usersearch WHERE name_ngram LIKE 'Al}%'
    UNION DISTINCT
    SELECT * FROM myapp.usersearch WHERE MATCH (username_ngram, name_ngram) AGAINST ('Al')  
) ng ON ng.user_id = ur.follows_id
WHERE ur.user_id = {user_idOfSearcher}
ORDER BY ??
LIMIT 10

Notes:

• I turned LEFT JOIN to an INNER JOIN because I think that's closer to what you want (you can change it back if it doesn't fit you requirement)

• You need an ORDER BY clause to go along with the LIMIT, otherwise the results are not deterministic when there are more that 10 rows in the resultset

来源：https://stackoverflow.com/questions/62479503/how-to-achieve-cte-functionality-in-mysql-5-7