Re: questions about 2nd index on one column

On 01/03/2017 11:35 AM, Ravi Kapoor wrote:
Please reply to list also.
Ccing list.

> Yes I am aware of django EOL. However, our company is still using it, we
> have a migration plan later this year, however for now, I got to work
> with what we have.

Still, you are missing 14 patch releases to the 1.5 version.

>
> Correct, the index will be on one column.
>
> In Django, the model is described in model.py file, so to update it, I
> will simply change following line
> votes1 = models.CharField(default='', max_length=200)
> to following line
> votes1 = models.CharField(db_index=True, default='', max_length=200)
>
> and run Django migrations.

Hmm, from this:

https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/commands/indexcmds.c;h=fdfa6ca4f5cd3be2db624e1c709ad037dc700b40;hb=c7681b2b9a115eb05048a485480826bc0efa6d3b

/*
* CheckIndexCompatible
* Determine whether an existing index definition is compatible with a
* prospective index definition, such that the existing index storage
* could become the storage of the new index, avoiding a rebuild.
*
/*

It would seem that the index would not be rebuilt, assuming all conditions are the same.

The part that has me questioning is this:

* This is tailored to the needs of ALTER TABLE ALTER TYPE, ..

Someone who knows the internals better will have to verify this.

>
>
> On Tue, Jan 3, 2017 at 11:26 AM, Adrian Klaver
> <adrian(dot)klaver(at)aklaver(dot)com <mailto:adrian(dot)klaver(at)aklaver(dot)com>> wrote:
>
> On 01/03/2017 11:07 AM, Ravi Kapoor wrote:
>
>
> I have a bit strange question. I am trying to figure out how to
> avoid
> table locking while creating an index through Django (1.5.1) in
> Postgres
> 9.4.7
>
> Django 1.5.1 does not support concurrent indexing. So my thought
> is to
> first create a concurrent index using SQL prompt.
> Then try to update django model to add index, which will effectively
> create 2 indexes on same column.
>
>
> I really need to read the entire subject. So in response to my own
> question, yes it is on one column.
>
>
>
> So my questions are:
>
> If I create a 2nd index on one column,
> 1. Does postgres scan entire table to create index from scratch
> or does
> it simply copy the first index?
> 2. Does postgres lock the table to create this index?
>
> thanks
>
>
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com <mailto:adrian(dot)klaver(at)aklaver(dot)com>
>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com