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Documentation of the system catalogs, directed toward PostgreSQL developers

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<chapter id="catalogs">

<entry><type>oid</type></entry>

<entry></entry>

<entry>

Lastoidinexistence afterthe database was created; useful

Lastsystem OIDin the database; useful

particularly to <application>pg_dump</application>

</entry>

</row>

<row>

<entry>datvacuumxid</entry>

<entry><type>xid</type></entry>

<entry></entry>

<entry>

All tuples inserted or deleted by transaction IDs before this one

have been marked as known committed or known aborted in this database.

This is used to determine when commit-log space can be recycled.

</entry>

</row>

<row>

<entry>datfrozenxid</entry>

<entry><type>xid</type></entry>

<entry></entry>

<entry>

All tuples inserted by transaction IDs before this one have been

relabeled with a permanent (<quote>frozen</>) transaction ID in this

database. This is useful to check whether a database must be vacuumed

soon to avoid transaction ID wraparound problems.

</entry>

</row>

<row>

<entry>datpath</entry>

<entry><type>text</type></entry>

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Postgres documentation

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simply by setting the flag false). The <literal>template0</literal>

database is normally marked this way to prevent modification of it.

</para>

<para>

After preparing a template database, or making any changes to one,

it is a good idea to perform

<command>VACUUM FREEZE</> or <command>VACUUM FULL FREEZE</> in that

database. If this is done when there are no other open transactions

in the same database, then it is guaranteed that all tuples in the

database are <quote>frozen</> and will not be subject to transaction

ID wraparound problems. This is particularly important for a database

that will have <literal>datallowconn</literal> set to false, since it

will be impossible to do routine maintenance <command>VACUUM</>s on

such a database.

See the Administrator's Guide for more information.

</para>

</refsect2>

</refsect1>

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</refnamediv>

<refsynopsisdiv>

<refsynopsisdivinfo>

<date>2001-07-10</date>

<date>2001-08-26</date>

</refsynopsisdivinfo>

<synopsis>

VACUUM [ FULL ] [ VERBOSE ] [ <replaceable class="PARAMETER">table</replaceable> ]

VACUUM [ FULL ] [ VERBOSE ] ANALYZE [ <replaceable class="PARAMETER">table</replaceable> [ (<replaceable class="PARAMETER">column</replaceable> [, ...] ) ] ]

VACUUM [ FULL ] [FREEZE ] [VERBOSE ] [ <replaceable class="PARAMETER">table</replaceable> ]

VACUUM [ FULL ] [FREEZE ] [VERBOSE ] ANALYZE [ <replaceable class="PARAMETER">table</replaceable> [ (<replaceable class="PARAMETER">column</replaceable> [, ...] ) ] ]

</synopsis>

<refsect2 id="R2-SQL-VACUUM-1">

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>FREEZE</term>

<listitem>

<para>

Selects aggressive <quote>freezing</quote> of tuples.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>VERBOSE</term>

<listitem>

</para>

<para>

Plain <command>VACUUM</command> simply reclaims space and makes it

<command>VACUUM ANALYZE</command> performs a <command>VACUUM</command>

and then an <command>ANALYZE</command> for each selected table. This

is a handy combination form for routine maintenance scripts. See

<xref linkend="sql-analyze" endterm="sql-analyze-title">

for more details about its processing.

</para>

<para>

Plain <command>VACUUM</command> (without <literal>FULL</>) simply reclaims

space and makes it

available for re-use. This form of the command can operate in parallel

with normal reading and writing of the table. <command>VACUUM

FULL</command> does more extensive processing, including moving of tuples

across blocks to try to compact the table to the minimum number of disk

blocks. This is much slower and requires an exclusive lock on each table

while it is being processed.

blocks. Thisformis much slower and requires an exclusive lock on each

tablewhile it is being processed.

</para>

<para>

<command>VACUUM ANALYZE</command> performs a <command>VACUUM</command>

and then an <command>ANALYZE</command> for each selected table. This

is a handy combination form for routine maintenance scripts. See

<xref linkend="sql-analyze" endterm="sql-analyze-title">

for more details about its processing.

<command>FREEZE</command> is a special-purpose option that

causes tuples to be marked <quote>frozen</quote> as soon as possible,

rather than waiting until they are quite old. If this is done when there

are no other open transactions in the same database, then it is guaranteed

that all tuples in the database are <quote>frozen</> and will not be

subject to transaction ID wraparound problems, no matter how long the

database is left un-vacuumed.

<command>FREEZE</command> is not recommended for routine use. Its only

intended usage is in connection with preparation of user-defined template

databases, or other databases that are completely read-only and will not

receive routine maintenance <command>VACUUM</> operations.

See <xref linkend="sql-createdatabase" endterm="sql-createdatabase-title">

for details.

</para>

<refsect2 id="R2-SQL-VACUUM-3">

{

intpageno;

if (TransactionIdToPgIndex(newestXact)!=0)

if (TransactionIdToPgIndex(newestXact)!=0&&

!TransactionIdEquals(newestXact,FirstNormalTransactionId))

return;

pageno=TransactionIdToPage(newestXact);

S_LOCK(&(ClogCtl->control_lck));

restart:;

if (CLOGPagePrecedes(ClogCtl->latest_page_number,cutoffPage))

{

S_UNLOCK(&(ClogCtl->control_lck));

elog(LOG,"unable to truncate commit log: apparent wraparound");

return;

}

for (slotno=0;slotno<NUM_CLOG_BUFFERS;slotno++)

{

TransactionLogUpdate(transactionId,TRANSACTION_STATUS_ABORTED);

}

TransactionIdPrecedes(TransactionIdid1,TransactionIdid2)

{

int32diff;

if (!TransactionIdIsNormal(id1)|| !TransactionIdIsNormal(id2))

return (id1<id2);

diff= (int32) (id1-id2);

return (diff<0);

}

TransactionIdPrecedesOrEquals(TransactionIdid1,TransactionIdid2)

{

int32diff;

if (!TransactionIdIsNormal(id1)|| !TransactionIdIsNormal(id2))

return (id1 <=id2);

diff= (int32) (id1-id2);

return (diff <=0);

}

TransactionIdFollows(TransactionIdid1,TransactionIdid2)

{

int32diff;

if (!TransactionIdIsNormal(id1)|| !TransactionIdIsNormal(id2))

return (id1>id2);

diff= (int32) (id1-id2);

return (diff>0);

}

TransactionIdFollowsOrEquals(TransactionIdid1,TransactionIdid2)

{

int32diff;

if (!TransactionIdIsNormal(id1)|| !TransactionIdIsNormal(id2))

return (id1 >=id2);

diff= (int32) (id1-id2);

return (diff >=0);

}

#definePG_GETARG_TRANSACTIONID(n)DatumGetTransactionId(PG_GETARG_DATUM(n))

#definePG_RETURN_TRANSACTIONID(x)return TransactionIdGetDatum(x)

xidin(PG_FUNCTION_ARGS)

{

char*representation=PG_GETARG_CSTRING(0);

char*str=PG_GETARG_CSTRING(0);

PG_RETURN_TRANSACTIONID((TransactionId)atol(representation));

PG_RETURN_TRANSACTIONID((TransactionId)strtoul(str,NULL,0));

}

xidout(PG_FUNCTION_ARGS)

{

TransactionIdtransactionId=PG_GETARG_TRANSACTIONID(0);

char*representation=palloc(11);

char*str=palloc(11);

snprintf(representation,11,"%lu", (unsigned long)transactionId);

snprintf(str,11,"%lu", (unsigned long)transactionId);

PG_RETURN_CSTRING(representation);

PG_RETURN_CSTRING(str);

}

xideq(PG_FUNCTION_ARGS)

PG_RETURN_BOOL(TransactionIdEquals(xid1,xid2));

}

xid_age(PG_FUNCTION_ARGS)

{

TransactionIdxid=PG_GETARG_TRANSACTIONID(0);

TransactionIdnow=GetCurrentTransactionId();

if (!TransactionIdIsNormal(xid))

PG_RETURN_INT32(INT_MAX);

PG_RETURN_INT32((int32) (now-xid));

}