[ovirt-devel] Writing SQL queries in DAO code

[Moti Asayag]

On Sun, Mar 26, 2017 at 5:12 PM, Yevgeny Zaspitsky <yzaspits at redhat.com>
wrote:

> Hi All,
>
> Recently I had a task of performance improvement in one of our network
> related flows and had some hard time following our DAL code and one of the
> outcomes of the task was defining a couple of new quite simple, but neat
> queries.
> When I came to coding those new queries I remembered how hard was
> following the existing DAL code, I decided to make my own new methods
> clearer. So I created [1] and [2] patches.
>
> Everything is quite standard there beside the fact that they do not use
> any stored procedure, but use SQL directly, IMHO by that they save time
> that I spent in trying to follow what a DAO method does. Looking into the
> method code you get the understanding of what this method is all about:
>
>    - no looking for a stored procedure name that is buried in the DAO
>    class hierarchy
>    - no looking for the SP definition
>
>
There are additional pros and cons for the suggestion to consider:

Pros:

   1. No need to run engine-setup after changing DB related code (in case
   of SQL inside Java).

Cons:

   1. DAO files might become very long.
   2. If you decide to return the business entity associated with the DAO
   as a returned object, you won't know as a caller which fields to expect to
   be populated, which lead to 3:
   3. An inflation of business entities to represent partial populated
   business entity or inflation of mappers inflation (this will be required
   for SP as well).
   4. SQL code inside of Java:
   1. Beside of the fact that a multi-line concatenated string that cannot
      be easily copied and run with psql, it means that we should compile the
      code in order to test the change (vs building with DEV_REBUILD=0
which only
      package the SQL file).
      2. No syntax highlighting when performing code review. i.e. I don't
      think reviewing a patch such as
      https://gerrit.ovirt.org/#/c/66729/10/packaging/dbscripts/network_sp.sql
      would be more clear inside a java file.
      3. The user permissions management is implemented on DB level. That
      means that SQL will be more complex (more concatenated java lines).
   5. Stored procedure are cached by project's code. See
   SimpleJdbcCallsHandler.getCall(), while the NamedParameterJdbcTemplate
   are cached by spring's code which seems less optimal (sync all calls using
   synchronized vs using ConcurrentHashMap as in SP cache).
   6. With the NamedParametersJdbcTemplate there is no use of the
   DbEngineDialect. What's the impact of it ?

So besides 5 and 6, the rest is a matter of style. I'd like to hear more
opinions from other members.

Regards,
Moti

So I'd like to propose moving towards this approach in general in all cases
> when nothing beyond a simple SQL is needed (no stored procedure programming
> language is needed).
> From my experience with the performance improvement task it looks like
> people avoid adding new queries for a specific need of a flow, instead they
> use the existing general ones (e.g. dao.getAllForX()) and do the actual
> join in the bll code.
> IMHO the proposed approach would simplify adding new specific queries and
> by that would prevent a decent part of performance issues in the future.
>
> I do not propose changing all existing SP's to inline queries in a once,
> but to allow adding new queries this way. Also we might consider converting
> relatively simple SP's to inline SQL statements later in a graduate way.
>
> [1] - https://gerrit.ovirt.org/#/c/74456
> [2] - https://gerrit.ovirt.org/#/c/74458
>
> Regards,
> Yevgeny
>
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-- 
Regards,
Moti
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