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Re^5: fetchall_arrayref hangs with placeholders query with no results

by mpeppler (Vicar)
on Apr 29, 2021 at 12:12 UTC ( #11131865=note: print w/replies, xml ) Need Help??


in reply to Re^4: fetchall_arrayref hangs with placeholders query with no results
in thread fetchall_arrayref hangs with placeholders query with no results

No - that syntax is indeed illegal in Sybase ASE (and I suspect MS-SQL).

This has nothing to do with the placeholder type - it's because the main point of placeholders is performance - it's so that you can re-use a SQL statement many times with different parameters without having to re-compile and re-optimize it. With a complex SQL query this can make a very big difference in performance (SQL parsing and optimization can take many 100s of milliseconds, much more that the actual fetching of the data on a large system).

With Sybase and MS-SQL what happens is that DBD::Sybase detects that one or more ?-style placeholders are in the SQL statement. This is then passed to the ct_dynamic() API, which performs the "prepare" step on the server, identifies the parameter types (based on the column types in the WHERE clause) and returns a handle to this prepared statement. This is now basically a temporary stored procedure.

DBD::Sybase has never done any client-side placeholder processing.

Personally I fail to see the point of using placeholders in the SELECT clause, unless there's some weird variable quoting issue that the OP wants to handle.

BTW - the DBI docs also state that ?-style placeholders will normally only work in WHERE or VALUES clauses: https://metacpan.org/pod/DBI#Placeholders-and-Bind-Values

Michael

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