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db-utf8-transcoder

A C program to transcode PostgreSQL string data to UTF8.

This project is targeted for PostgreSQL 9.0 running on Ubuntu 10.0.4 LTS. Ports to other PostgreSQL versions and OS's are welcome!

The transcoder runs against one table at a time. It does the following things:

  1. Gets the schema and table to convert, plus other options (report only, force conversion, print debug messages, help) from the command line.
  2. Opens read and write connections to the database.
  3. Dynamically determines the shortest (in terms of columns) unique index for the table.
  4. Dynamically determines the character-based columns for the table.
  5. Get the minimum value from the unique index.
  6. Gets the row for that minimum value.
  7. For each character-based column in the row: a. detects the encoding, if it can, b. converts the value from the detected encoding to UTF8. If the encoding cannot be determined for a particular character string the original bytestream is returned.
  8. Writes the converted and/or unconverted data back to the same row.
  9. Logs the conversion once the update is confirmed.
  10. Cleans up memory and database connections, and exits.

Note steps 6, 7 and 8 are not transactional, as they are running on two different connections and the SELECT of the values in step 6 does not use a READ FOR UPDATE lock. This is purposeful, so the transcoder doesn’t block other activity in the database. The (very, very) small risk here is a row could be updated between steps 6 and 8, and the update could be lost when the row is overwritten in step 8.

Variability in the number of rows converted per second for different tables (or even the same table) is due to at least three conditions:

  1. the number of columns to convert,
  2. the size of data in the columns to convert,
  3. and the concurrent load on the database and host machine.

DANGER, WILL ROBINSON! DANGER!

CHARACTER SET DETECTION IS NOT NECESSARILY DETERMINISTIC, PARTICULARLY FOR LATIN-BASED LANGUAGES. THE DETECTOR MAY MAKE THE WRONG GUESS AND TRANSCODE YOUR DATA INCORRECTLY! BE SURE TO FOLLOW PRUDENT PROCEDURES TO PROTECT YOUR DATA, INCLUDING BUT NOT LIMITED TO BACKUPS, HISTORY TABLES, AND THROWAWAY DATABASES!

YOU HAVE BEEN WARNED!

DBA note:

Watch for enforced autovacuums due to wraparound transaction protection when updating large numbers of rows. Also concurrently reindex any indexed columns that may have been updated after the run completes.

To run:

Run the transcoder on the database host from the postgres login using a command line like so:

transcoder --dsn='dbname=<db> user=postgres' --schema=<schema> --table=<table> 1> >(gzip > /tmp/transcoder-runs/<table>.out.gz)  2> >(gzip > /tmp/transcoder-runs/<table>.err.gz)

This will compress the stdout and stderr streams, which can get quite large for tables with millions of rows. To tail the logs use:

zcat /tmp/transcoder-runs/<table>.err.gz | tail -f

To build:

Build and install ICU libraries and header files

To use the latest ICU libraries following the directions below. The ICU libraries will be installed in /usr/local/lib. The header files will be installed in /usr/local/include.

wget http://download.icu-project.org/files/icu4c/54.1/icu4c-54_1-src.tgz
tar -xzf icu4c-54_1-src.tgz
cd icu/source
mkdir -p ./data/out/build/icudt54l
mkdir -p ./data/out/tmp
./runConfigureICU --enable-debug Linux/gcc
make && sudo make install

You can also simply install the required packages, but the transcoder was built and tested using ICU 54.1:

sudo apt-get -y apt-get install libicu42 libicu-dev libicu42-dbg

Build db-utf8-transcode executable

Install necessary PostgreSQL packages for build

sudo apt-get install -y -qq postgresql-9.0
sudo apt-get install -y -qq postgresql-9.0-dbg
sudo apt-get install -y -qq postgresql-contrib-9.0
sudo apt-get install -y -qq libpq-dev
sudo apt-get install -y -qq postgresql-server-dev-9.0

Update library path

sudo ldconfig

Build and install transcoder executable. The executable will be in /usr/local/bin.

mkdir pg-utf8-transcoder
cd pg-utf8-transcoder
git clone https://github.com/aweber/pg-utf8-transcoder.git
./configure
make clean && make && sudo make install
Install the db functions

Install the db functions used by the transcoder into the target db.

sudo su - postgres
cd pg-utf8-transcoder/sql
for f in $(ls -1 *.sql); do psql <targetdb> -f $f; done
Testing

Create a test database.

sudo su - postgres
createdb test

Install the db functions used by transcoder.

sudo su - postgres
cd pg-utf8-transcoder/sql
for f in $(ls -1 *.sql); do psql test -f $f; done

Install the test data set.

cd pg-utf8-transcoder/test-data
zcat test.dump_p.gz | psql test

Convert the data in text.

transcoder --dsn='dbname=test' \
          --schema=public \
          --table=test \
          2> /tmp/transcoder-test.err \
          1> /tmp/transcoder-test.out

Inspect the output in the latest /tmp/run_transcoder.* for notices, warnings and errors. There should be no errors, but may be warnings if a conversion is not possible.