Re: WIP: Covering + unique indexes.

On Fri, Jan 26, 2018 at 3:01 AM, Anastasia Lubennikova
<a(dot)lubennikova(at)postgrespro(dot)ru> wrote:
> Thanks for the reminder. Rebased patches are attached.

This is a really cool and also difficult feature. Thanks for working
on it! Here are a couple of quick comments on the documentation,
since I noticed it doesn't build:

SGML->XML change: (1) empty closing tags "</>" are no longer accepted,
(2) <xref ...> now needs to be written <xref .../> and (3) xref IDs
are now case-sensitive.

+ PRIMARY KEY ( <replaceable
class="parameter">column_name</replaceable> [, ... ] ) <replaceable
class="parameter">index_parameters</replaceable> <optional>INCLUDE
(<replaceable class="parameter">column_name</replaceable> [,
...])</optional> |

I hadn't seen that use of "<optional>" before. Almost everywhere else
we use explicit [ and ] characters, but I see that there are other
examples, and it is rendered as [ and ] in the output. OK, cool, but
I think there should be some extra whitespace so that it comes out as:

[ INCLUDE ... ]

instead of:

[INCLUDE ...]

to fit with the existing convention.

+ ... This also allows <literal>UNIQUE</> indexes to be defined on
+ one set of columns, which can include another set of columns in the
+ <literal>INCLUDE</> clause, on which the uniqueness is not enforced.
+ It's the same with other constraints (PRIMARY KEY and
EXCLUDE). This can
+ also can be used for non-unique indexes as any columns which
are not required
+ for the searching or ordering of records can be used in the
+ <literal>INCLUDE</> clause, which can slightly reduce the
size of the index.

Can I suggest rewording these three sentences a bit? Just an idea:

<literal>UNIQUE</literal> indexes, <literal>PRIMARY KEY</literal>
constraints and <literal>EXCLUDE</literal> constraints can be defined
with extra columns in an <literal>INCLUDE</literal> clause, in which
case uniqueness is not enforced for the extra columns. Moving columns
that are not needed for searching, ordering or uniqueness into the
<literal>INCLUDE</literal> clause can sometimes reduce the size of the
index while retaining the possibility of using a faster index-only
scan.

--
Thomas Munro
http://www.enterprisedb.com