Re: Add pg_walinspect function with block info columns

On Tue, Mar 7, 2023 at 12:48 PM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>
> On Tue, Mar 07, 2023 at 03:49:02PM +0900, Kyotaro Horiguchi wrote:
> > Ah. Yes, that expansion sounds sensible.
>
> Okay, so, based on this idea, I have hacked on this stuff and finish
> with the attached that shows block data if it exists, as well as FPI
> stuff if any. bimg_info is showed as a text[] for its flags.

+1.

> I guess that I'd better add a test that shows correctly a record with
> some block data attached to it, on top of the existing one for FPIs..
> Any suggestions? Perhaps just a heap/heap2 record?
>
> Thoughts?

That would be a lot better. Not just the test, but also the
documentation can have it. Simple way to generate such a record (both
block data and FPI) is to just change the wal_level to logical in
walinspect.conf [1], see code around REGBUF_KEEP_DATA and
RelationIsLogicallyLogged in heapam.c

I had the following comments and fixed them in the attached v2 patch:

1. Still a trace of pg_get_wal_fpi_info in docs, removed it.

2. Used int4 instead of int for fpilen just to be in sync with
fpi_length of pg_get_wal_record_info.

3. Changed to be consistent and use just FPI or "F/full page".
/* FPI flags */
/* No full page image, so store NULLs for all its fields */
/* Full-page image */
/* Full page exists, so let's save it. */
* and end LSNs. This produces information about the full page images with
* to a record. Decompression is applied to the full-page images, if

4. I think we need to free raw_data, raw_page and flags as we loop
over multiple blocks (XLR_MAX_BLOCK_ID) and will leak memory which can
be a problem if we have many blocks assocated with a single WAL
record.
flags = (Datum *) palloc0(sizeof(Datum) * bitcnt);
Also, we will leak all CStringGetTextDatum memory in the block_id for loop.
Another way is to use and reset temp memory context in the for loop
over block_ids. I prefer this approach over multiple pfree()s in
block_id for loop.

5. I think it'd be good to say if the FPI is for WAL_VERIFICATION, so
I changed it to the following. Feel free to ignore this if you think
it's not required.
if (blk->apply_image)
flags[cnt++] = CStringGetTextDatum("APPLY");
else
flags[cnt++] = CStringGetTextDatum("WAL_VERIFICATION");

6. Did minor wordsmithing.

7. Added test case which shows both block data and fpi in the documentation.

8. Changed wal_level to logical in walinspect.conf to test case with block data.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com