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`DISTINCT` support #253

Closed

LordPython opened this Issue Apr 2, 2016 · 0 comments

LordPython commented Apr 2, 2016

There isn't currently support for creating SELECT DISTINCT queries.

sgrif added a commit that referenced this issue Apr 2, 2016


      Add support for the `DISTINCT` keyword

A few things to note about the implementation. I've opted for 2 unit
structs instead of something like an enum or boolean so that the code
gets inlined via monomorphisation instead of containing a branch (e.g.
if you don't use distinct, it's free). As always, it's enforced that our
query builder doesn't do "real work" by
https://github.com/sgrif/diesel/blob/c8127d408f759faa056503014cd00e92479215e0/diesel_tests/tests/perf_details.rs#L8-L13

This ended up churning a lot of `select_dsl`. I've reformatted the code
surrounding what was touched to reduce churn in the future.

This doesn't yet add support for `DISTINCT ON(expr)`, as that is a PG
specific feature and can be added as a separate PR.

An alternative API for this would be something like
`users.select(distinct(name))`. I actually think I might prefer that
API, but actually enforcing correctness with it (making sure `distinct`
can only be passed to `select` and not something like `filter` --
disallowing things like `max(distinct(id))`, etc) would be pretty much
impossible.

Fixes #253.

d2e2b8c

sgrif referenced this issue Apr 2, 2016

Merged

Add support for the `DISTINCT` keyword #255

sgrif added a commit that referenced this issue Apr 2, 2016


      Add support for the `DISTINCT` keyword

A few things to note about the implementation. I've opted for 2 unit
structs instead of something like an enum or boolean so that the code
gets inlined via monomorphisation instead of containing a branch (e.g.
if you don't use distinct, it's free). As always, it's enforced that our
query builder doesn't do "real work" by
https://github.com/sgrif/diesel/blob/c8127d408f759faa056503014cd00e92479215e0/diesel_tests/tests/perf_details.rs#L8-L13

This ended up churning a lot of `select_dsl`. I've reformatted the code
surrounding what was touched to reduce churn in the future.

This doesn't yet add support for `DISTINCT ON(expr)`, as that is a PG
specific feature and can be added as a separate PR.

An alternative API for this would be something like
`users.select(distinct(name))`. I actually think I might prefer that
API, but actually enforcing correctness with it (making sure `distinct`
can only be passed to `select` and not something like `filter` --
disallowing things like `max(distinct(id))`, etc) would be pretty much
impossible.

Fixes #253.

9e88bab

sgrif added a commit that referenced this issue Apr 6, 2016


      Add support for the `DISTINCT` keyword

A few things to note about the implementation. I've opted for 2 unit
structs instead of something like an enum or boolean so that the code
gets inlined via monomorphisation instead of containing a branch (e.g.
if you don't use distinct, it's free). As always, it's enforced that our
query builder doesn't do "real work" by
https://github.com/sgrif/diesel/blob/c8127d408f759faa056503014cd00e92479215e0/diesel_tests/tests/perf_details.rs#L8-L13

This ended up churning a lot of `select_dsl`. I've reformatted the code
surrounding what was touched to reduce churn in the future.

This doesn't yet add support for `DISTINCT ON(expr)`, as that is a PG
specific feature and can be added as a separate PR.

An alternative API for this would be something like
`users.select(distinct(name))`. I actually think I might prefer that
API, but actually enforcing correctness with it (making sure `distinct`
can only be passed to `select` and not something like `filter` --
disallowing things like `max(distinct(id))`, etc) would be pretty much
impossible.

Fixes #253.

7c6042c

sgrif closed this in #255 Apr 6, 2016

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