pg_prewarm.c

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/*-------------------------------------------------------------------------
 *
 * pg_prewarm.c
 *        prewarming utilities
 *
 * Copyright (c) 2010-2025, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *        contrib/pg_prewarm/pg_prewarm.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include <sys/stat.h>
#include <unistd.h>
 
#include "access/relation.h"
#include "fmgr.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/read_stream.h"
#include "storage/smgr.h"
#include "utils/acl.h"
#include "utils/builtins.h"
#include "utils/lsyscache.h"
#include "utils/rel.h"
 
PG_MODULE_MAGIC_EXT(
                    .name = "pg_prewarm",
                    .version = PG_VERSION
);
 
PG_FUNCTION_INFO_V1(pg_prewarm);
 
typedef enum
{
    PREWARM_PREFETCH,
    PREWARM_READ,
    PREWARM_BUFFER,
} PrewarmType;
 
static PGIOAlignedBlock blockbuffer;
 
/*
 * pg_prewarm(regclass, mode text, fork text,
 *            first_block int8, last_block int8)
 *
 * The first argument is the relation to be prewarmed; the second controls
 * how prewarming is done; legal options are 'prefetch', 'read', and 'buffer'.
 * The third is the name of the relation fork to be prewarmed.  The fourth
 * and fifth arguments specify the first and last block to be prewarmed.
 * If the fourth argument is NULL, it will be taken as 0; if the fifth argument
 * is NULL, it will be taken as the number of blocks in the relation.  The
 * return value is the number of blocks successfully prewarmed.
 */
Datum
pg_prewarm(PG_FUNCTION_ARGS)
{
    Oid         relOid;
    text       *forkName;
    text       *type;
    int64       first_block;
    int64       last_block;
    int64       nblocks;
    int64       blocks_done = 0;
    int64       block;
    Relation    rel;
    ForkNumber  forkNumber;
    char       *forkString;
    char       *ttype;
    PrewarmType ptype;
    AclResult   aclresult;
 
    /* Basic sanity checking. */
    if (PG_ARGISNULL(0))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("relation cannot be null")));
    relOid = PG_GETARG_OID(0);
    if (PG_ARGISNULL(1))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("prewarm type cannot be null")));
    type = PG_GETARG_TEXT_PP(1);
    ttype = text_to_cstring(type);
    if (strcmp(ttype, "prefetch") == 0)
        ptype = PREWARM_PREFETCH;
    else if (strcmp(ttype, "read") == 0)
        ptype = PREWARM_READ;
    else if (strcmp(ttype, "buffer") == 0)
        ptype = PREWARM_BUFFER;
    else
    {
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("invalid prewarm type"),
                 errhint("Valid prewarm types are \"prefetch\", \"read\", and \"buffer\".")));
        PG_RETURN_INT64(0);     /* Placate compiler. */
    }
    if (PG_ARGISNULL(2))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("relation fork cannot be null")));
    forkName = PG_GETARG_TEXT_PP(2);
    forkString = text_to_cstring(forkName);
    forkNumber = forkname_to_number(forkString);
 
    /* Open relation and check privileges. */
    rel = relation_open(relOid, AccessShareLock);
    aclresult = pg_class_aclcheck(relOid, GetUserId(), ACL_SELECT);
    if (aclresult != ACLCHECK_OK)
        aclcheck_error(aclresult, get_relkind_objtype(rel->rd_rel->relkind), get_rel_name(relOid));
 
    /* Check that the fork exists. */
    if (!smgrexists(RelationGetSmgr(rel), forkNumber))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("fork \"%s\" does not exist for this relation",
                        forkString)));
 
    /* Validate block numbers, or handle nulls. */
    nblocks = RelationGetNumberOfBlocksInFork(rel, forkNumber);
    if (PG_ARGISNULL(3))
        first_block = 0;
    else
    {
        first_block = PG_GETARG_INT64(3);
        if (first_block < 0 || first_block >= nblocks)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("starting block number must be between 0 and %" PRId64,
                            (nblocks - 1))));
    }
    if (PG_ARGISNULL(4))
        last_block = nblocks - 1;
    else
    {
        last_block = PG_GETARG_INT64(4);
        if (last_block < 0 || last_block >= nblocks)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("ending block number must be between 0 and %" PRId64,
                            (nblocks - 1))));
    }
 
    /* Now we're ready to do the real work. */
    if (ptype == PREWARM_PREFETCH)
    {
#ifdef USE_PREFETCH
 
        /*
         * In prefetch mode, we just hint the OS to read the blocks, but we
         * don't know whether it really does it, and we don't wait for it to
         * finish.
         *
         * It would probably be better to pass our prefetch requests in chunks
         * of a megabyte or maybe even a whole segment at a time, but there's
         * no practical way to do that at present without a gross modularity
         * violation, so we just do this.
         */
        for (block = first_block; block <= last_block; ++block)
        {
            CHECK_FOR_INTERRUPTS();
            PrefetchBuffer(rel, forkNumber, block);
            ++blocks_done;
        }
#else
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("prefetch is not supported by this build")));
#endif
    }
    else if (ptype == PREWARM_READ)
    {
        /*
         * In read mode, we actually read the blocks, but not into shared
         * buffers.  This is more portable than prefetch mode (it works
         * everywhere) and is synchronous.
         */
        for (block = first_block; block <= last_block; ++block)
        {
            CHECK_FOR_INTERRUPTS();
            smgrread(RelationGetSmgr(rel), forkNumber, block, blockbuffer.data);
            ++blocks_done;
        }
    }
    else if (ptype == PREWARM_BUFFER)
    {
        BlockRangeReadStreamPrivate p;
        ReadStream *stream;
 
        /*
         * In buffer mode, we actually pull the data into shared_buffers.
         */
 
        /* Set up the private state for our streaming buffer read callback. */
        p.current_blocknum = first_block;
        p.last_exclusive = last_block + 1;
 
        /*
         * It is safe to use batchmode as block_range_read_stream_cb takes no
         * locks.
         */
        stream = read_stream_begin_relation(READ_STREAM_FULL |
                                            READ_STREAM_USE_BATCHING,
                                            NULL,
                                            rel,
                                            forkNumber,
                                            block_range_read_stream_cb,
                                            &p,
                                            0);
 
        for (block = first_block; block <= last_block; ++block)
        {
            Buffer      buf;
 
            CHECK_FOR_INTERRUPTS();
            buf = read_stream_next_buffer(stream, NULL);
            ReleaseBuffer(buf);
            ++blocks_done;
        }
        Assert(read_stream_next_buffer(stream, NULL) == InvalidBuffer);
        read_stream_end(stream);
    }
 
    /* Close relation, release lock. */
    relation_close(rel, AccessShareLock);
 
    PG_RETURN_INT64(blocks_done);
}