Document predicate pushdown support for string-type columns in SQL Se…

[Jessie212]

…rver

Description

Document predicate pushdown support for string-type columns.

Additional context and related issues

#15714

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# Section
* Fix some things. ({issue}`issuenumber`)

[Jessie212]

@hashhar I need your help with this.

[mosabua]

I have no idea what that means. What setting do you change from what to what?

[vlad-lyutenko]

In SqlServer every textual column have a Collation.
Collation it's kind of set of rules how to treat text symbols in this column during different operations (like compare during sorting).
For example collation Latin1_General_CI_AS - means for this column we translate bytes to symbols using Latin byte to symbol translation and CI - mean Case Insensitivity and other rules.
So our code is working only if collation is Case Sensitive for column.

[vlad-lyutenko]

Case sensitive collation means that in Collation name there is part like CS or BIN or BIN2

for example: Latin1_General_CS_AS

[hashhar]

What do you think about wording like:

The connector supports pushdown of predicates on ``VARCHAR`` and
``NVARCHAR`` columns if the underlying columns in SQL Server use a
case-sensitive collation. The following operators are pushed down:

- ``=``
- ``<>``
- ``IN``
- ``NOT IN``

If the column in SQL Server uses a case-insensitive collation then no
operators are pushed down to preserve correct results. To enable
pushdown on such columns you must change the collation to be a
case-sensitive one in SQL Server. See https://learn.microsoft.com/en-us/sql/relational-databases/collations/collation-and-unicode-support?view=sql-server-ver16

The TL;DR is:

• SQL Server column is case-sensitive
  • =, !=, IN and NOT IN can be pushed down
  • <, >, BETWEEN etc. cannot be pushed down
• SQL Server column is case-insensitive
  • No pushdown at all unless you change the collation in SQL Server to be a case-sensitive one

[Jessie212]

Thanks for taking time to explain, @hashhar.

[hashhar]

LGTM % comment

[hashhar]

Suggested change

	Operators are not pushed down and correct results are not preserved for columns
	using a case-insensitive collation.
	Operators are not pushed down for columns using a case-insensitive collation
	to preserve correct results.

the current sentence reads the opposite (i.e. results are incorrect)

[Jessie212]

It's the "to preserve correct results" that needs to be changed to sound less awkward. Since I'm having trouble improving the flow of the sentence without changing the meaning, please help me understand this process better.

For columns using a case-insensitive collation, operators are not pushed down, which preserves correct results. Is that right or wrong?

[hashhar]

For columns using a case-insensitive collation, operators are not pushed down, which preserves correct results.

Yes, this is the behaviour and intention.

[mosabua]

Looks good.