Re: Massive table (500M rows) update nightmare

At 08:17 07/01/2010, Carlo Stonebanks wrote:
>Our DB has an audit table which is 500M rows and growing. (FYI the objects being audited are grouped semantically, not
individualfield values). 
>
>Recently we wanted to add a new feature and we altered the table to add a new column. We are backfilling this
varchar(255)column by writing a TCL script to page through the rows (where every update is a UPDATE ... WHERE id >= x
ANDid < x+10 and a commit is performed after every 1000 updates statement, i.e. every 10000 rows.) 
>
>We have 10 columns, six of which are indexed. Rough calculations suggest that this will take two to three weeks to
completeon an 8-core CPU with more than enough memory. 
>
>As a ballpark estimate - is this sort of performance for an 500M updates what one would expect of PG given the table
structure(detailed below) or should I dig deeper to look for performance issues? 
>
>As always, thanks!
>
>Carlo

You can dump the table with pg_dumpand get a file like this (only a few columns, not all of them). Note that the last
line,has the data in TSV (Tab Separate Format) plus a LF. 

SET statement_timeout = 0;
SET client_encoding = 'UTF8';
SET standard_conforming_strings = off;
SET check_function_bodies = false;
SET client_min_messages = warning;
SET escape_string_warning = off;

SET search_path = public, pg_catalog;

SET default_tablespace = '';

SET default_with_oids = false;

CREATE TABLE mdx_core.audit_impt
(
 audit_impt_id serial NOT NULL,
 impt_session integer,
 impt_version character varying(255),
}

COPY mdx_core.audit_impt (audit_impt_id, impt_session, impt_version) FROM stdin;
1       1       tateti
2       32      coll

You can add a new column hacking it, just adding the new column to the schema, the name in the copy statement and a
tabulator+dataat the end of the line (before the LF). Note that the table name is different from the original. 

SET statement_timeout = 0;
SET client_encoding = 'UTF8';
SET standard_conforming_strings = off;
SET check_function_bodies = false;
SET client_min_messages = warning;
SET escape_string_warning = off;
SET search_path = public, pg_catalog;
SET default_tablespace = '';
SET default_with_oids = false;
CREATE TABLE mdx_core.audit_impt2
(
 audit_impt_id serial NOT NULL,
 impt_session integer,
 impt_version character varying(255),
 source_table character varying(255)
}
COPY mdx_core.audit_impt2 (audit_impt_id, impt_session, impt_version, source_table) FROM stdin;
1       1       tateti  tentown
1       32      coll    krof


After this, add indexes, constraints as usual.

HTH

--------------------------------
Eduardo Morrás González
Dept. I+D+i e-Crime Vigilancia Digital
S21sec Labs
Tlf: +34 902 222 521
Móvil: +34 555 555 555
www.s21sec.com, blog.s21sec.com


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