In python, how can I load a sqlite db completely t

问题:

This question already has an answer here:

• How to load existing db file to memory in Python sqlite3? 8 answers

I have a 100 mega bytes sqlite db file that I would like to load to memory before performing sql queries. Is it possible to do that in python?

Thanks

回答1:

apsw is an alternate wrapper for sqlite, which enables you to backup an on-disk database to memory before doing operations.

From the docs:

###
### Backup to memory
###

# We will copy the disk database into a memory database

memcon=apsw.Connection(":memory:")

# Copy into memory
with memcon.backup("main", connection, "main") as backup:
    backup.step() # copy whole database in one go

# There will be no disk accesses for this query
for row in memcon.cursor().execute("select * from s"):
    pass

connection above is your on-disk db.

回答2:

1. Get an in-memory database running (standard stuff)
2. Attach the disk database (file).
3. Recreate tables / indexes and copy over contents.
4. Detach the disk database (file)

Here's an example (taken from here) in Tcl (could be useful for getting the general idea along):

proc loadDB {dbhandle filename} {

    if {$filename != ""} {
        #attach persistent DB to target DB
        $dbhandle eval "ATTACH DATABASE '$filename' AS loadfrom"
        #copy each table to the target DB
        foreach {tablename} [$dbhandle eval "SELECT name FROM loadfrom.sqlite_master WHERE type = 'table'"] {
            $dbhandle eval "CREATE TABLE '$tablename' AS SELECT * FROM loadfrom.'$tablename'"
        }
        #create indizes in loaded table
        foreach {sql_exp} [$dbhandle eval "SELECT sql FROM loadfrom.sqlite_master WHERE type = 'index'"] {
            $dbhandle eval $sql_exp
        }
        #detach the source DB
        $dbhandle eval {DETACH loadfrom}
    }
}

回答3:

Note that you may not need to explicitly load the database into SQLite's memory at all. Simply prime your operating system disk cache by copying it to null.

Windows: copy file.db nul:
Unix/Mac:  cp file.db /dev/null

This has the advantage of the operating system taking care of memory management, especially discarding it if something more important comes along.

回答4:

If you are using Linux, you can try tmpfs which is a memory-based file system.

It's very easy to use it:

1. mount tmpfs to a directory.
2. copy sqlite db file to the directory.
3. open it as normal sqlite db file.

Remember, anything in tmpfs will be lost after reboot. So, you may copy db file back to disk if it changed.