sql-server - 计算列上的索引触发 900 字节索引大小限制

Question

我有一个包含计算得出的 VARCHAR 列的表，该列最多包含 106 个字符:

CREATE TABLE report (
    report_id INT IDENTITY(1, 1) NOT NULL,
    name VARCHAR(100) COLLATE Modern_Spanish_CI_AI NOT NULL,
    city_id VARCHAR(6) COLLATE Modern_Spanish_CI_AI,

    unique_name AS
    CASE
        WHEN city_id IS NULL
        THEN name
        ELSE name + REPLICATE(' ', 100 - LEN(name)) + city_id
    END COLLATE Modern_Spanish_CI_AI,

    CONSTRAINT report_pk PRIMARY KEY (report_id)
);

/* Report name is unique per city (and among city-less rows) */
CREATE UNIQUE INDEX report_idx1 ON report (unique_name);

但是当我运行该语句时，我收到此警告:

Warning! The maximum key length is 900 bytes. The index 'report_idx1' has maximum length of 8000 bytes. For some combination of large values, the insert/update operation will fail.

有没有办法告诉 SQL Server 该列不会超过 106 个字符，以便我消除警告？

Answer 1

尝试CAST(CASE ... END AS VARCHAR(106))...

CAST(CASE
    WHEN city_id IS NULL
    THEN name
    ELSE name + REPLICATE(' ', 100 - LEN(name)) + city_id
END AS VARCHAR(106)) COLLATE Modern_Spanish_CI_AI 

或者干脆忽略它...这只是一个警告。