Re: [PATCH] bitmap indexes

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Abhijit Menon-Sen <ams(at)2ndquadrant(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org, bausch(at)dvs(dot)tu-darmstadt(dot)de
Subject: Re: [PATCH] bitmap indexes
Date: 2013-09-25 22:20:17
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Here are some quick items while skimming this patch. I am looking at
commit 6448de29d from your github repo, branch bmi.

What's with the pg_bitmapindex stuff in pg_namespace.h? It doesn't seem
to be used anywhere.

This led me to research how these indexes are stored. I note that what
we're doing here is to create another regular table and a btree index on
top of it, and those are the ones that actually store the index data.
This seems grotty and completely unlike the way we do things elsewhere
(compare GIN indexes which have rbtrees inside them). I think this
should be stored in separate forks, or separate kinds of pages
intermixed in the index's main fork.

I don't like that files are named bitmap.c/.h. We already have bitmap
scans, so having the generic concept (bitmap) show up as a file name is
confusing. To cite an example, see the name of the bmgetbitmap function
(which returns a TID bitmap from a bitmap index. How is that not
confusing?). I think I would be more comfortable with the files being
called "bmi" or maybe "bitmapidx", or something like that. (For sure do
not change the AM's name. I mean "CREATE INDEX .. USING bmi" would

Not happy with (the contents of) src/include/access/bitmap.h. There's
way too much stuff in a single file. I think a three-file split would
work nicely: one for the AM routine declarations (bitmap.h), one for
xlog stuff (bitmap_xlog.h), one for internal routines and struct
declarations (bitmap_priv.h, bitmap_internals.h, or something like
that). Also, I think macros and structs should be defined in a narrow
scope as possible; for example, macros such as HEADER_SET_FILL_BIT_ON
should be defined in bitmaputil.c, not bitmap.h (that macro is missing
parens BTW). For macros that are defined in headers, it would be better
to have prefixes that scope them to bitmaps; for example IS_FILL_WORD
should maybe have a BM_ prefix or something similar.

I don't think it's necessary to renumber relopt_kind. Just stash the
new one at the end of the enum.

bmoptions's DESCR entry in pg_proc.h says "btree".

contrib/bmfuncs.c defines CHECK_PAGE_OFFSET_RANGE but doesn't seem to
use it anywhere.

same file defines CHECK_RELATION_BLOCK_RANGE using a bare { } block. Our
style is to have these multi-statement macros use do {} while (false).

#include lines in source files are normally alphabetically sorted. The
new code fails to meet this expectation in many places.

First four lines of _bitmap_init seem gratuitous ..

All those #ifdef DEBUG_BMI lines sprinkled all over the place look
pretty bad; they interrupt the indentation flow. I would suggest to
define a macro or some macros to emit debugging messages, which are
enabled or disabled in a single place depending on DEBUG_BMI. Something
simple such as DO_DB() in fd.c would suffice.

I don't like this comment one bit: /* misteriously, MemSet segfaults... :( */
I think that's probably a bug that should be investigated rather than
papered over.

I don't understand the cur_bmbuild thingy .. I think that stuff should
be passed down as arguments to whatever do the index build, instead of
being a magical global var that also signals failure to find hash
functions for some datatypes.

Above definition of BMBuildHashData there's a comment referring us to
execGrouping.c but I cannot understand what it refers to.

"block unfound"? In general I think it's poor style to spell out the
function name in error messages. I mean, ereport() already reports the
function name. Also, please don't split long error messages across
multiple lines; better to leave the line to run off the screen.

I'm unsure about distinguishing errors in the recovery routines that
raise ERROR from those that PANIC. I mean, they would both cause the
server to PANIC.

The bitmap_xlog_cleanup routine talks about an "uninitialized reln".
That's a large point about xlog recovery -- you don't have relations.
You only have relfilenodes. I think you need to shuffle some routines
so that they can work with only a relfilenode.

Generally, the xlog stuff needs a lot of comments.

A pgindent run would be beneficial.

Álvaro Herrera
PostgreSQL Development, 24x7 Support, Training & Services

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