Sql-server – Storing Live Timeseries Data in SQL Server Azure Database

First scenario, it's scanning the whole table and looking through 14553 pages of data. Second scenario, it's doing 2266 seeks (but counted as range scans) which each look at just 2 pages. So the second one is way better. Plus, many of those seeks will probably be looking at pages which have just been looked at, so on a cold cache it will be an even larger performance benefit. And, the second is more likely to parallelise better, being lots of small operations rather than a large one (which could still be parallelised, but it's more effort).

Your numbers table is a heap and is potentially being fully scanned each time.

Add a clustered primary key on Number and try the following with a forceseek hint to get the desired seek.

As far as I can tell this hint is needed as SQL Server just estimates that 27% of the table will match the predicate (30% for the <= and reduced down to 27% by the <>). And therefore that it will only have to read 3-4 rows before finding one that matches and it can exit the semi join. So the scan option is costed very cheaply. But in fact if any palindromes do exist then it will have to read the whole table so this is not a good plan.

CREATE FUNCTION dbo.InlineIsPalindrome
(
    @Word NVARCHAR(500)
)
RETURNS TABLE
WITH SCHEMABINDING
AS RETURN (
    WITH Nums AS
    (
      SELECT
        N = number
      FROM
        dbo.Numbers WITH(FORCESEEK)
    )
    SELECT
      IsPalindrome =
        CASE
          WHEN EXISTS
          (
            SELECT N
            FROM Nums
            WHERE N <= L / 2
              AND SUBSTRING(S, N, 1) <> SUBSTRING(S, 1 + L - N, 1)
          )
          THEN 0
          ELSE 1
        END
    FROM
      (SELECT LTRIM(RTRIM(@Word)), LEN(@Word)) AS v (S, L)
);
GO

With those changes in place it flies for me (takes 228ms)