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<concept id="GUID-84921F68-9449-564A-8527-203FA84AF2AF" xml:lang="en"><title>Database

compression</title><shortdesc>A Store database can be compressed and the data is dynamically

decompressed as required. A compressed database is read-only.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>A <xref href="GUID-13C95985-D16F-5A9E-A7F8-CAB637C4C6ED.dita">Store database</xref> can

be compressed.</p>

<p>Once compressed, a Store database is read-only, but otherwise operates

as normal and is opened in exactly the same way as an uncompressed database;

data is dynamically decompressed as required.</p>

<p>The memory required to decompress data as it is read, depends on the length

of the streams in the database store, but is typically just a few kilobytes.

A compressed database is not noticably slower than the equivalent uncompressed

database, especially on slower media, such as CF-card.</p>

<p>The <codeph>RDbStoreDatabase</codeph> class provides the functions <codeph>CompressL()</codeph> and <codeph>DecompressL()</codeph> to

convert a Store database to compressed form and vice-versa.</p>

<p>Note that:</p>

<ul>

<li id="GUID-0DD719CA-9073-5F03-8BB0-C71BE702E957"><p>encrypted databases

cannot be compressed</p> </li>

<li id="GUID-BE9AD15E-3B20-56CF-AFF4-4528457881C3"><p>index data is not compressed

as the presence of indexes reduces the effective compression ratio</p> </li>

<li id="GUID-D782E99C-AE1E-5158-9F5D-FD9ED94C6B29"><p>compression code on

the emulator is more aggressive than that on ARM builds and can achieve better

compression ratios; the resulting files are about 5% smaller at the expense

of doing two or three times as much work</p> </li>

<li id="GUID-7DA7C65C-C0CB-5C9E-810C-7318D7AC48EA"><p>compressing or decompressing

a database can require a significant amount of memory.</p> </li>

</ul>

<p>When a file-store database is constructed using the <codeph>RDbNamedDatabase</codeph> interface,

there is no access to the underlying store</p>