diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/storage.sgml | 15 |
1 files changed, 9 insertions, 6 deletions
diff --git a/doc/src/sgml/storage.sgml b/doc/src/sgml/storage.sgml index 8a2e7929e62..1973a5b90c3 100644 --- a/doc/src/sgml/storage.sgml +++ b/doc/src/sgml/storage.sgml @@ -1,4 +1,4 @@ -<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.15 2007/03/02 00:48:44 tgl Exp $ --> +<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.16 2007/04/03 04:14:26 tgl Exp $ --> <chapter id="storage"> @@ -240,8 +240,9 @@ of the LZ family of compression techniques. See <para> Out-of-line values are divided (after compression if used) into chunks of at -most <literal>TOAST_MAX_CHUNK_SIZE</> bytes (this value is a little less than -<literal>BLCKSZ/4</>, or about 2000 bytes by default). Each chunk is stored +most <symbol>TOAST_MAX_CHUNK_SIZE</> bytes (by default this value is chosen +so that four chunk rows will fit on a page, making it about 2000 bytes). +Each chunk is stored as a separate row in the <acronym>TOAST</> table for the owning table. Every <acronym>TOAST</> table has the columns <structfield>chunk_id</> (an OID identifying the particular <acronym>TOAST</>ed value), @@ -260,10 +261,12 @@ regardless of the actual size of the represented value. <para> The <acronym>TOAST</> code is triggered only -when a row value to be stored in a table is wider than <literal>BLCKSZ/4</> -bytes (normally 2 kB). The <acronym>TOAST</> code will compress and/or move +when a row value to be stored in a table is wider than +<symbol>TOAST_TUPLE_THRESHOLD</> bytes (normally 2 kB). +The <acronym>TOAST</> code will compress and/or move field values out-of-line until the row value is shorter than -<literal>BLCKSZ/4</> bytes or no more gains can be had. During an UPDATE +<symbol>TOAST_TUPLE_TARGET</> bytes (also normally 2 kB) +or no more gains can be had. During an UPDATE operation, values of unchanged fields are normally preserved as-is; so an UPDATE of a row with out-of-line values incurs no <acronym>TOAST</> costs if none of the out-of-line values change. |