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Re: [WebSQLDatabase] Adding a vacuum() call

From: Dumitru Daniliuc <dumi@chromium.org>
Date: Tue, 16 Mar 2010 16:50:00 -0800
Message-ID: <e753f47f1003161750vc96c466lc3d24b3d961300eb@mail.gmail.com>
To: shane@sqlite.org
Cc: public-webapps <public-webapps@w3.org>
shane, i was hoping you could clarify a few things about AUTO_VACUUM:

> However, the B-Tree balancing algorithm used by SQLite will attempt to
> merge pages with neighbors when there space utilization drops below certain
> thresholds. Minimum average fill for intkey leaves is 50%. For other pages I
> think it's 66%.

according to http://www.sqlite.org/pragma.html#pragma_auto_vacuum:
"Auto-vacuum does not defragment the database nor repack individual database
pages the way that the VACUUM command does."

so the way i understand these statements is:
1. sqlite always attempts to merge mostly-empty pages. the (auto-)vacuum
settings have no effect on that.
2. AUTO_VACUUM only moves pages around and deletes the empty ones. it does
not try to repack individual pages. however, because of #1, page repacking
happens anyway (to some extent), when AUTO_VACUUM is on.

am i right?

and one more question:

"Auto-vacuuming is only possible if the database stores some additional
information that allows each database page to be traced backwards to its
referer. Therefore, auto-vacuuming must be turned on before any tables are
created. It is not possible to enable or disable auto-vacuum after a table
has been created."

what happens if we create a database without AUTO_VACUUM on, insert some
data, save it to a file, then turn on AUTO_VACUUM and try to open that
database again? will sqlite add the missing information? will AUTO_VACUUM be
silently turned off for that database? will we get an error when we try to
open/read from/write to that database? anything else that we need to be
aware of in this case?

Received on Wednesday, 17 March 2010 00:50:33 UTC

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