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TRUNCATE+COPY optimization and --jobs=1 in pg_restore

From: Takahiro Itagaki <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
To: pgsql-hackers(at)postgresql(dot)org
Subject: TRUNCATE+COPY optimization and --jobs=1 in pg_restore
Date: 2010-02-10 02:56:54
Message-ID: 20100210115654.47E4.52131E4D@oss.ntt.co.jp (view raw or flat)
Thread:
Lists: pgsql-hackers
We have an optimization to bulkload date in pg_restore, but the code
only works in parallel restore (--jobs >= 2). Why don't we do the
same optimization in the serial restore (--jobs = 1) ?

We checks is_parallel to decide to use BEGIN-TRUNCATE-COPY:
    if (is_parallel && te->created)
but we can always do it unless --single-transaction, right?
    if (!ropt->single_txn && te->created)


[ in restore_toc_entry() ]
/*
 * In parallel restore, if we created the table earlier in
 * the run then we wrap the COPY in a transaction and
 * precede it with a TRUNCATE. If archiving is not on
 * this prevents WAL-logging the COPY. This obtains a
 * speedup similar to that from using single_txn mode in
 * non-parallel restores.
 */
if (is_parallel && te->created)     <==== HERE
{
    /*
     * Parallel restore is always talking directly to a
     * server, so no need to see if we should issue BEGIN.
     */
    StartTransaction(AH);

    /*
     * If the server version is >= 8.4, make sure we issue
     * TRUNCATE with ONLY so that child tables are not
     * wiped.
     */
    ahprintf(AH, "TRUNCATE TABLE %s%s;\n\n",
             (PQserverVersion(AH->connection) >= 80400 ?
              "ONLY " : ""),
             fmtId(te->tag));
}


Regards,
---
Takahiro Itagaki
NTT Open Source Software Center


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