900 byte index size limit in character length

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The storage size for varchar is the actual length of the data entered + 2 bytes. Even though the column itself has that 2 byte overhead, you can put up to 900 byte varchar values into a column which is indexed.

In practice, you can create an index on a column larger than 900 bytes in size, but you will have a problem if you actually try to insert something larger than 900 bytes:

create table test (
    col varchar(1000)
);
create index test_index on test (col);
-- Warning! The maximum key length is 900 bytes. The index 'test_index' has maximum length of 1000 bytes. For some combination of large values, the insert/update operation will fail.
insert into test select cast(replicate('x', 899) as varchar(1000)); -- Success
insert into test select cast(replicate('y', 900) as varchar(1000)); -- Success
insert into test select cast(replicate('z', 901) as varchar(1000)); -- Fail
-- Msg 1946, Level 16, State 3, Line 8
-- Operation failed. The index entry of length 901 bytes for the index 'test_index' exceeds the maximum length of 900 bytes.

Be aware that the 900-byte limit includes all columns of a given index key, as this example shows:

create table test (
      col varchar(1000)
    , otherCol bit -- This column will take a byte out of the index below, pun intended
);
create index test_index on test (col, otherCol);
insert into test select cast(replicate('x', 899) as varchar(1000)), 0; -- Success
insert into test select cast(replicate('y', 900) as varchar(1000)), 0; -- Fail
insert into test select cast(replicate('z', 901) as varchar(1000)), 0; -- Fail

For these columns that are normally too large for an index key, you may be able to gain some benefits of indexing by including them in an index.

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